Top tip: how not to lose out on a game in the Instant Game Collection.
You can add all the games in the Instant Game Collection to your shopping cart and check out. That will mean that even after a game is removed from the Instant Game Collection, you will still have access to it through your Transaction History and Download List.
To make it even easier, you can now do this, straight from your web browser (I like to do it this way, I find the store to be sluggish when accessed from the console).
https://store.sonyentertainmentnetwork.com/