I used to do the same thing, especially before PayPal shipping labels. The Stamp shows that I went to, you had to buy $300 face value to get the 20% discount to $240. I wallpapered a lot of packages with 3, 4, & 5 cent stamps that some of the dealers would give a 25% discount on, as folks didn't want to mess with putting 20+ stamps on a letter/package.