Is the postage for USPS first class envelope-typed package the same as its box-typed on eBay?

[aselling81521]

The 2021 system: 

I’ve noticed that there’s no setup to enter package details for USPS first class ENVELOPE as I tried to print a shipping label today on eBay. On the package details screen, there’s only weight and dimension (L*W*H) criteria that need to round up to the nearest ones. Saying, enter the least whole number 1 for each criteria, 1x1x1’’, that’s already a box, although it’s not proper to ship in that small size. It looks like the system assumes all the custom size packages are of box, which excludes the envelope/mailer type. I’m fine with the system that does not distinguish the first class package by its types: either box or envelope/poly mailer.

The reason I brought it up is that when I use the eBay shipping calculator, which distinguishes envelope type from box, for reference on my postage, I end up realizing it costs me MORE to ship a light weight item with USPS first class without identifying its package types. Specifically, when entering the same packaging details and destination information and such both for shipping calculator and on shipping label screen, the USPS first class envelope type costs $1-3, but the box type costs $3.01+. So ebay shipping calculator is misleading for envelope-typed package postage.  

Anyone with the same question/doubt on this?

[nobody*s_perfect]

USPS has 3 types of First Class mail: letters, flats (AKA Large envelopes), and packages.

An "envelope" could fall into any of those 3 categories, depending on factors like weight, dimensions (including thickness), rigidity etc.

eBay only sells labels for packages. If your envelope meets the USPS criteria to be mailed as a letter or flat, then you will have to purchase postage at the Post Office, or just put stamps on it.

BTW, dimensions do not affect postage cost for any eBay labels, except for the dimensional weight for packages over 1 cubic foot (1728 cubic inches). So entering to the nearest inch is sufficient for eBay's purposes. If you are mailing an envelope as a First Class package, and if it's less than 1 inch thick, just enter "1" as the thickness dimension.