jecasc - 6:31 pm on Feb 6, 2013 (gmt 0)
First of all: The customer has a contract with you when he shops online - the credit card is just a method of payment, a means for the customer to fulfill his part of the contract.
So the question would not be, what is in the contract between the customer and the credit card company - that does not concern you, but what does the contract you have with the credit card or the payment provider say.
The credit card company can't simply give the customer the right to cancel an order without you being involved in some way. For example the credit card company could say: If you want to accept our credit card as means of payment you have to agree to abide to a certain policies about order cancellations.
Otherwise if no such agreements exist: You could very well insist the customer abides to the contract.
The reason customers are able to issue chargebacks in the first place is often: They simply lie about the reason for the chargeback.
A good source of information for VISA is here:
Especially the download "Chargeback Management Guidelines".
In the European Union however things are different than in the US, because of the distance selling regulations private individuals have the right to cancel most online orders anyway.