Quality Park Bubble Mailers, 6 x 9 Shipping Envelopes, Self-Seal, Durable Brown Kraft Padded Envelopes, Redi-Strip Peel Off Closure, 50 Per Box (QUA85755)

I similarly ship 6”x9” padded mailers (these) almost every day (nothing heavy or thick inside, literally just a trading card in a plastic toploader). The last few times I’ve shipped them directly via USPS—in person at the Post Office—shipping has been over $8. Ground Advantage, no signature or added insurance, continental 48. When I purchase labels directly from eBay, with their discount, postage ranges from $5 to $7. What might I be doing wrong?