How are they getting 20% on a deposit that presumably could be called up at any time, and how can I get in on it when the stupid "High Yield" accounts I can find top out at around 4%?
probably doesn't come up to 20% (unless Starbucks is in junk bond territory) but it's higher than the investment rate of 4% that you're quoting.
The numbers given have to be incorrect.
Most of the time they have a buried clause that says that you forfeit all of your credit or get charged an inactivity fee if there have been no account transactions or no credit added for 12 or 18 months. Same reason why you should never buy gift cards.
So they can keep collecting interest on it forever, but they have to keep the balance for you indefinitely.
By law, gift cards never expire.
Unfortunately, I have several gift cards I didn't use before the store expired.