Why is USPS First Class Letter rate not calculating correctly?

[discovery]

Everything I ship qualifies for the USPS First Class Letter rate which is currently about $0.68. My letters are 5" x 7" or 6" x 9" and under .25 inches thick so they should cost the rate for letters, but the automatically calculated rate always shows the rate for flats which is $1.30. I have set the package size and thickness correctly and set it to USPS First Class Mail for envelopes. How can I get the correct rate?

[Mirdas]

What you are seeing is the difference between letters and large envelopes.

Letters:

• Standard-sized, rectangular envelope stamps start at $0.68.

Large Envelopes:

• Large envelopes (flats) start at $1.39.

It is unclear whether Shopify Shipping supports letters.

[NWCG]

We found that the ShipBlink app works well for First Class Letter at the commercial rate (0.64), and it has the added bonus of being based on EasyPost, which has free letter tracking. Also the support by the developer is awesome if you run into any issues during your trial.

[discovery]

Thanks for this suggestion!