FYI, the
ebay standard envelope is tracked by the USPS sorting machines. They read the code and that automatically adds an entry like "origin processing" and "destination processing". But there is never a manual scan made by a person. The algorithm marks them as delivered sometimes, but there is no actual delivery scan.
I use them all the time for cheap sales (<=$20). Been working fine. I probably sent 100 or so in total.
I want to report one bit of good news. I had a stamp lost in the mail around Thanksgiving and refunded the buyer in December. It arrived today and he is returning it to me. This was a couple hundred dollar item, 4 months somehow reappeared - he said it looks unopened. Hoping it is true.