Where to put 40 million coins?

The only way to get rid of them is to spend them, man!  They sure do rack up fast once you've gotten every sticker you can get from packs.  Rux will give you opportunity to use some of them.

If you somehow got them by accident, as you suggest, I really think it would be silly to quit the game just because you've got a lot of coins.  Just go crazy opening packs, you can get a very large way through the sticker book with 40M coins.

Do you have kids? You might want to check your bank account.