Shopping Cart payment system issues:
If your card is getting rejected it likely is the security on your card provided by your bank. Often times your bank will text you or call you to verify it is actually you making the purchase. Sometimes it is the merchant processor. This is because my original merchant account was for my last business (Fern River Resort).  There is left over code from my past merchant account somewhere in Google or other location that nobody can find to correct.
Certain banks see this and call an error thus blocking the transaction. The fix is: Use a different card or call your bank. Your bank will than approve the charge. Sorry, this is the new world where everyone is trying to stop scammers.