Naturally the user is always responsible for the purchase he/she makes. However, if I (OP) make my selections and need to re-enter some information based on their request - I don't think that allows them to change my decisions from what I have chosen earlier, especially if you get an error message similar to "please check your credit card number".

It is also a de facto standard in general that sites do not change once set selections when fixing errors in the original form and once you differ from this you're bound to increase burden on your customer support.