Lob Help Center


Letters overview

To get started with sending letters, check out our Letters API documentation, GitHub library, or Template Gallery for inspiration. 

Dimensions & specs

Lob offers 8.5" x 11" letters in both black & white and in color, which can be printed single or double-sided. Letters and checks not exceeding 6 tri-folded pages in total will be sent in standard #10 double-windowed outer envelopes. Letters and checks over 6 pages but not exceeding 60 total pages will be mailed in a flat envelope. 

Reference our design templates on where to place your design elements for letters.

We use a few pre-approved papers across our commercial printer network. In ensuring uniform quality and consistency across all of our mail pieces, each paper source specification must fall within a small range. The specification/value range pairs are: 

  • 8.5" x 11" B&W & color letters
    • Basis Weight: 60# text
    • GSM: 89 - 90

Address placement

The address_placement parameter specifies the location of the address information that will show through the double-window envelope. Options are top_first_page and insert_blank_page.

By default, Lob selects address_placement = top_first_page, meaning Lob will print address information at the top of your provided first page. Make sure to leave ample space for the address information to be printed, which will show through our standard double-windowed envelope. To see how this will impact your letter design, view our letter template.

If you pass address_placement = insert_blank_page, a blank address page will be inserted at the beginning of your file where we will print the address information to show through our standard double-windowed envelope. Note that you will be charged for the extra page. Reference the Letter Unit Pricing section in our Pricing page for charges you will incur based on your tier. 

If you don't follow these address guidelines, your letter will be printed incorrectly.

Mailing letters

Ensure your letters get to their final destination by following these guidelines:

  • The from field is required for all letters, regardless of the destination
  • Letters may contain PII as envelopes have an interior security tint for privacy
  • Mailing Class options may differ based on the destination and use case (e.g. promotional vs transactional)

Certified Mail & Registered Mail

Certified Mail with return receipts

Certified Mail is an add-on service offered by USPS for First Class mail that provides proof of mailing to the sender. With electronic USPS tracking, the sender is notified when the mailing was delivered or that a delivery attempt was made. An electronic return receipt showing the recipient's signature is also available for Certified Mail only. Both the Certified Mail and the electronic return receipt add-ons are only available for First Class mail sent domestically within the US.

You send letters as Certified Mail through the Lob API by passing the value certified in the extra_services parameter of the create letter request. You may similarly opt for an electronic return receipt by passing certified_return_receipt in the extra_services parameter in the API.

Tracking events are viewable directly within the Lob dashboard or via the API, similar to how they are for non-Certified letters, and you can also subscribe to webhook events for Certified letters. When you make a request for Certified Mail you will immediately receive a carrier tracking number (retrievable via your Lob dashboard), which can be used to track the mail via the carrier’s website - or you can choose to track the mail via the scan events within your Lob dashboard. 

The resulting screen should appear in your dashboard:

Note that tracking events for Certified Mail differ from tracking events received for other mail types (see the full list in our API documentation for Certified tracking event details). Electronic return receipts are accessible via the certified tracking number from the USPS homepage, but the signature PDF can be requested to be emailed to you. 

Visit the Certified Mail envelopes section for more guidance around sending Certified Mail.

Registered Mail

Registered Mail is an add-on service offered by USPS that provides extra protection for high-value letters and packages. When you send a letter by Registered Mail, the USPS establishes a chain of custody that tracks and secures your shipment throughout the entire transit process—from the moment it is dropped off at the Post Office until the moment it’s delivered.

You can send letters as Registered Mail through the Lob API by passing the value registered in the extra_services parameter of the create letter request.

The USPS Registered Mail add-on is only available for First Class mail and can be sent and tracked both domestically and internationally. 

Letters sent as registered will receive a different set of scan events compared to regular First or Standard Class mail. Registered Mail will instead receive a carrier tracking number and link, which is an add-on that will be available 3 business days following the mailer’s send_date. This tracking number can be used to track the mailer via the carrier’s website.

Add-on: Cards

Paper cards can now be affixed to letters, providing a compelling method to direct customers to special promotions and drive engagement, both online and in-store. Having “faux” cards can serve as a tangible and memorable reminder for any upcoming marketing promotion.   

To get started with letters with card affix, check out our API documentation or visit our Template Gallery for inspiration.


Enterprise feature

Affixing cards to letters is a feature that is exclusive to our Enterprise-tier customers. Upgrade to the appropriate Print & Mail edition to gain access, or reach out to our sales team to learn more.

Dimensions & specs

Paper cards can only be affixed horizontally to the top fold of a tri-folded letter, and to the face letter on the first page. Card affixed letters are limited to a single page in total (but can be double-sided), and will be sent in a standard #10 double-windowed envelope. Custom envelopes are not supported for card-affixed letters at this time.

Reference our layout templates below on where to place your design elements on the card, and the location of where the card will be affixed to any given letter.

Card specs

  • 2.125” x 3.375” card, with 0.125” rounded corners
  • 18-24pt paper thickness, depending on paper availability
  • Coated 2 sides (gloss varnish) + full bleed
  • Affixing adhesive: medium tack fugitive glue

Card design & guidelines

Artwork can be featured on both sides of the card. In general, any critical design elements or text should be 0.125" away from the final trim line due to movement at press. There are also no color limitations for cards. Other considerations include:

  • Double-check that all images are embedded into your document
  • Ensure all rasterized artwork (images, effects, copy, etc.) is created and saved at or above 300 dpi
  • Thin, small fonts with over 3 colors may fill in slightly or appear “fuzzy”
  • Line weights of 0.5 pt or more assure optimum print results
  • Any barcodes/QR codes provided need to be print-ready
  • CMYK document

Refer to our design guides for more details on image prepping, or browse our Template Gallery for pre-designed creatives to leverage as inspiration.


‍Ordering cards

Cards must be created, ordered, and printed before they can be utilized. Upload your front and back designs to create a new card, then order inventory for your card. The minimum order quantity is 10,000 cards per artwork design submitted, and will be invoiced at the time of order. Cards can be purchased as a one-off order or through auto-reordering.

Order cards via the Lob dashboard

  • Go to the Cards section in the Lob dashboard and hit the Create button
  • Upload a front and back design, and hit Create
  • Once card details show, select the Order button at the bottom
  • Add the desired number of cards and hit Order

Order cards via the Cards API

  • Send a request to the /v1/cards endpoint with the following fields:
    • Description (Required, string): A name to identify your card
    • Front (Required, string): A locally hosted PDF or hosted PDF URL for the front card artwork
    • Back (Optional, string): A locally hosted PDF or hosted PDF URL for the back card artwork

  • Send a request to the /v1/cards/{id}/orders endpoint with the following field:
    • Quantity (Required, number): Number of cards you would like to order


Order lead times 

Allow an estimated 20 business days for cards to be available for use in your Lob dashboard. Cards can not be used until the dashboard indicates that they have been made available, and any API requests including your unique Card ID will fail. Timing and delivery is dependent on order size and complexity, and may be additionally delayed by forces outside of Lob's control (e.g. USPS delays, printer site shutdowns due to Covid, extreme weather events).

Affixing cards to letters

Once cards are ordered, they cannot be sent with letters until your Lob dashboard indicates they are fully stocked and available in your inventory. An email will be sent once your cards are in stock. If a letter is created with a specified card ID that is not in stock, the letter request will be rejected. Affixing charges will be billed with each Letter API call made, which is separate from the initial card order. 

Affix cards to letters via the Lob dashboard

Coming soon!


Affix cards to letters via the Letters API

Currently, affixing cards to letters can only be done via the API, similar to Custom Envelopes.

  • Set the cards parameter in your POST call to https://api.lob.com/v1/letters to the desired card ID, which is found in your Lob dashboard
  • Additionally, you can find all cards in your account by sending a GET call to https://api.lob.com/v1/cards


Minimum send quantity

Cards must be sent in groups of at least 5,000 card-affix letter API requests during any 24-hour print-day period (10AM PT to 10AM PT). If you do not, you may incur flat setup costs of $250/day on your monthly usage invoice. Reach out to your dedicated Customer Success Manager if you have questions.

Card inventory management

Return to the Cards section in the Lob dashboard to view existing order designs at any given time and to manage your inventory for specific card designs.

Card details

View the number of remaining cards and the number of orders that are still outstanding or fulfilled. Inventory will decrease with each letter API call made that specifies a card design.

  • The available quantity will decrement as you send card-affixed letters, and increment if you cancel a letter request. It may take a few minutes for the number to update after any action. 
  • Alternatively, use a GET call to the https://api.lob.com/v1/cards endpoint to return a list of all cards on your account, and any available and pending inventory.


Given cards require additional lead times, we recommend enabling auto-reordering for cards that will be continuously utilized to ensure there will be no risk of running out. When reordering is turned on, a new order will be submitted whenever the remaining inventory quantity falls below 20%. A confirmation will show the reorder quantity and price that will be charged. 

  • Select the card design you would like to auto-reorder
  • Go to the Auto-Reorder Settings section, hit Set Up, set auto-reorder to on, and hit Save
  • Add a reorder quantity, the number of cards to be ordered when your most recent order falls below 20% of the original order quantity, and hit Save
  • To turn off auto-reordering, go into the same Auto-Reorder Settings and set the auto-reorder button to off and hit Save

Order history

Order details will be populated in the order history window one submitted, including order date, Card ID, order quantity, and inventory availability. The estimated date provides a rough estimate of when the cards will be available; however, an email confirmation will be sent when the cards are actually ready for use.

Card affix offering & variations

Currently we provide the following card affix offering:

  • Cards affixed to letters (one page total, front-side only)
  • Static, non-personalized artwork designs for paper cards
  • Horizontal card orientation in a single size
  • Cards affixed to the top fold of a trifold letter towards the right
  • Letters sent in standard #10 outer envelopes

We do not support the following offerings:

  • Cards affixed to any other form factor (e.g. self-mailers)
  • Cards affixed to letters that totals more than a single page, or on the back-side
  • Dynamic, personalized artwork designs that are unique to the recipient
  • Plastic cards (e.g. credit or loyalty cards)
  • Vertical card orientation or different card sizes
  • Cards affixed to any desired location on a letter
  • Letters sent in customized #10 outer envelopes
  • Card affix for Registered or Certified letters

Add-on: Buckslips

Coming soon!

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