Now go back to the USPS site and read the part at the end with the words *parcel only and then note that the * is next to words first class in the section you are reading.
First class letters do NOT have tracking. First class parcels do.
Since tracking is required for eBay transactions the label services available on eBay, PayPal, Auctiva will not offer first class letter postage as there is no tracking available for it.
Standard Post (retail only)
* Parcels only
And since you know how to make a thick envelope then you should also know it ships at the parcel rate not the letter rate. Letters are flat. Thick envelopes ship at parcel rates.