Letters and Flats are USPS Modern mail classes for lightweight mail, such as letters and large envelopes. Each class has strict size and weight limits, and Letters and Flats has limits when you rate shop.
For every USPS Modern service level, see Overview: USPS Modern Integration with ShipHero. To connect your USPS Modern account, see Connecting USPS Modern to ShipHero.
Table of Contents
- USPS Modern Letters and Flats Dimensional Requirements
- USPS Modern Letters and Flats Best Practices
- USPS Modern Letters and Flats Limitations
USPS Modern Letters and Flats Dimensional Requirements
USPS Modern applies strict dimensional rules to the Letters and Flats mail classes. Check each package against the limits below.
| Mail Class | Dimensions (Length × Height × Thickness) | Max Weight |
|---|---|---|
| Letters | 5-11.5" × 3.5-6.125" × 0.007-0.25" | Up to 3.5 oz |
| Flats | 6-15" × 3.5-12" × 0.25-0.75" | Up to 13 oz |
USPS Modern Letters and Flats Best Practices
These practices help you ship Letters and Flats with USPS Modern without size or rate shopping problems.
- Use the USPS Modern boxes from the Shipping Boxes page to confirm your package dimensions are within the USPS Modern requirements. Print the barcodes to scan the box type at the packing station, or use automation rules to pre-set the box on orders as they are imported.
- Rate shopping is available for Letters and Flats. However, map these shipping methods directly to your orders with automation rules or on the Shipping Methods page.
USPS Modern Letters and Flats Limitations
Letters and Flats is not a time-bound service in USPS Modern, which affects when it appears in rate shopping.
- ShipHero considers Letters and Flats when you rate shop with the Cheapest Ever shipping method configuration. It does not consider Letters and Flats for the Cheapest time-bound options, such as Cheapest 2-day and Cheapest 4-day.
- If you use RateShop+ with a Maximum Delivery Time set, ShipHero does not return Letters and Flats options.