If the customer receives a decline message or error at checkout there is an issue with either the funds on their account or the information that they have entered with the associated credit card.
As security around e-commerce tightens for credit card transactions more information is being checked that is associated with the card itself. Some credit card processors will check card number, expiration date, CVV, address and/or zip code that is connected to the card. Many were only checking card number, expiration date and CVV code originally.
If they are using a Saved card on an account have the customer delete the card and enter the information again.
If ALL cards are being declined including a test run be the merchant email firstname.lastname@example.org don't forget to tell us your Merchant Name or number and Location Name or number