You say 'getting greedy', and I say 'getting out of competition with credit/debit card thieves who can offer Amazon purchases for almost any discount and still profit with near-zero expenses'...
Let's call the whole thing off!
Do you know that for sure?
Do you know how purse can offer 20%? Are they stealing cards?
Theories:
Purse itself as opposed to its sellers, is buying cards from Amazon at wholesale discount. See all those cards in retail stores, and the codes that Gyft Coinstars spit out? There's no added fee for buying them, which means they have to be getting them at a discount to be making a profit/enough for operating expenses.
Amazon Associates referral fees might be involved too.