It has only accepted paypal for like... the entirety of the game's lifespan
Just create an account, hook up the card, complete the purchase and remove the card?
as long as you remember the gmail connected to the account (google play does that(?)) you can go to https://forum.supermechs.com/forum/ucp.php?mode=sendpassword&battlegate_bmm_sid=684e5aae7c6f89fb77fcadacc592791f and type it in