1) "Buy" the 9 free titles today (links at the link). Get 900 coins ($9 credit).
2) Use 699 coins ($6.99) to buy GTA: San Andreas.
3) You have 201 ($2.01) left over plus you get an addition 2000 coins ($20) for buying GTA.
Net result = free 2201 coins ($22.01) as credit to use at the Appstore.
This works because you're stacking promos, one of which is only valid for today.
Note that the version of GTA:SA is Kindle only, so if you don't have a Kindle you can't play it, but the coin credit works on pretty much all the Android apps on the Appstore.
You could do it all from PC browser itself.
Found it via GAF: