Been doing this for the last 15 years with no real change in pricing (which really means a loss every year).
Mostly 30-50 cent stamps (this is Canada). But ranging from 8c to $2.5
I use the smaller ones on regular envelopes. Usually I have some $1 or $2 stamps to make up any balance on packages (or just buy it from the post office).
It’s quite a bit of extra profit when I sell stuff.
Though my packages probably look a little... Unabombery.
I’m sure the receivers get a little laugh when their package arrives covered in stamps.