Why are my first class mails being returned due to insufficient funds?

Payments + Shipping & Fulfilment

Topic summary

Returned First-Class mail is being reported for a very small, lightweight item shipped in a 7 x 8 inch bubble mailer, even though Shopify’s shipping calculator shows a $1 rate. The seller wants to know whether the underpayment issue is caused by incorrect Shopify pricing or by a USPS error.

Key details provided:

  • Product size/weight: about 4 x 4 x 0.05 inches, roughly 0.05–0.08 oz.
  • Packaging: 7 x 8 x 0.25 inch bubble mailer.
  • Example route: ZIP 75068 to 63301.
  • Quoted service: First-Class Mail at $1.

The main unresolved question is whether a bubble mailer qualifies for the quoted letter/flat rate, or whether USPS is treating it as a different mail class that costs more. The seller is also asking for a specific postal rule or policy they can reference if the post office is applying the charge incorrectly.

No resolution or confirmed explanation is included in the discussion provided.

Summarized with AI on March 11. AI used: gpt-5.4.
1

I am having some serious shipping issues.

I have a small and light product. Measuring 4" x 4" x 0.05". 0.05oz weight . I ship in a 7" X 8" bubble mailer. When I calculate the cost in Shopify the price is $1 first class mail but my mails are being returned to me as insufficient funds. I also checked the Shopify web app calculator

https://www.shopify.com/toolbox/shipping/calculator and rate is the same as in Shopify $1 for first class mail.

My question is. Why is my mails being returned? is the Shopify pricing incorrect? or is the post office incorrect? if the post office is incorrect is there a specific policy that I can point to so that I can take to the post office?

Here is an example shipment:

Shipping From 75068 to 63301
Package size: 7 x 8 x 0.25 inches bubble mailer
Package weight: 0.08 OZ