Can't buy Camp Keepalive? Thread

I keep getting the message "We were unable to complete your payment, you have not been charged. Please try again." when trying to buy Camp Keepalive.

The credit card (actually check card) I am using is used frequently. I use it regularly for both local purchases and online purchases, and have done both recently and since when I started getting the error, so it's not the card.

Any advice? Thanks.