I've decided I'm going to do this too, and I think Sam's is best.
They're eVGCs, so they can't be used in store; only online.
What I don't like about these small fixed denomination cards is you have to under pay and be left with a small balance, or if you buy something more expensive than the balance then most shopping carts won't let you use an additional credit card; you'll have to split payment with a gift card.
Sam's allows you to split payment on several (3?) credit cards, so you can use up the $200 and $100 pretty easily in one transaction with 2x $150 DGC packs (2x$144.98=$289.96), and I'm thinking just load the remaining balance $10.04 balance to
Amazon or something. Or, you could spend the combined $300 towards a $500 DGC (priced at $484.98), but you'd need to cover the additional $184.98. You could buy a $200
Walmart eGC from Gyft (3x UR with CIP through PayPal checkout, 5x UR on CIC through PayPal checkout), but the lower 3x or 5x return will lower your overall % discount, unless you happen to be working on an MSR for these cards; and then you'll still be left with a $15.02 Walmart/Sam's credit.