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

[LERJR]

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