Why you need a stiffener for one stamp is beyond me. If your envelope is
under an ounce, the extra nonmachinable cost is still only 20 cents,
making your total 66 cents.
I'd guess you haven't [yet] gotten the mangled 'regular' envelopes,
let alone had to use a PO box. Delight seem to be taken to crease in
processing and stack, lean the small items, then cram in junk mails
which fold or crumple 'letter' mails.

I don't use eBay so I don't need to track stamps.
Considering the paucity of dealers handling my interest in this area -
actually there are NO dealers within 90 miles period, and NONE with GB
within 140 miles - eBay or similar is needed to get the better or
unusual items.