Discrepancy from USPS to Shopify shipping rates

[StitchWitch7]

I had a package returned to me for insufficient postage, and I can't seem to fix my shipping settings so that I pay for the label what USPS says it's supposed to cost.  I've sent orders of the same weight in the same package from the USPS office for $5.10. The rates I'm being shown on the USPS website are for that same price, and I have triple checked the weight and package measurements. When I go to buy and print a shipping label through Shopify, it's showing me that it will cost $3.90. I can't figure out how to get it to quote me an accurate price for the label without putting in the wrong weight, and I don't want to do that either because I don't know if they'll send it back a second time if it says it's heavier than it actually is. Is it the discounted rate through Shopify that's causing this, (is is really that much)? Did I mess up a setting somewhere? Please help!