I know the USPS discounts stamps that it sells to it's contract stations and Costco. I don't know if it's a 5% or 10% discount, but they could be coming from someone able to get them at wholesale pricing.

The max discount that you could get on them, that I'm aware of is 2-2.5%, from cashback on a card, although if a promo came out, maybe as much as 5%.