Did you already close the sales contract?
What do your TOS state when the sales contract is closed, what does the law in your jurisdiction say?
Usually a sales contract is closed when someone offers something and the other accepts. My TOS for example state that submitting an order is the offer, by shipping the goods I accept the contract.
Or you can state that you accept the offer in a separate email, or by charging the credit card. You can also do it the other way round: That what you have on your website is the offer and the customer accepts by submitting the order.
However if you have not thought about this and have made no provisions for this in your TOS, then a court would look at the circumstances.
For example If your customer submitted the order and you sent an email stating: Thank you very much for your order, we will deliver on January XY. A court could conclude that with this email you accepted the offer and already closed a contract.
A payment is not necessary to close a contract - but already part of fulfilling the contract.
Once the contract is closed it is usually binding - unless you have made provisions in your TOS that state when someone can cancel. And unless this TOS are even valid in your jursdiction (which is often not the case). Often there are also differences between B2B and B2C contracts in the relevant laws.