A declined payment means the customer's bank didn't approve the transaction — the customer hasn't been charged.
What to do
Ask the customer to try again.
Suggest a different card, or a phone wallet like Apple Pay or Google Pay.
Offer another method — QR code or cash.
Declines are decided by the customer's bank, so there's often nothing wrong on your end. If a customer is repeatedly declined, they should check with their bank.