Hello Colin,
The key to all of this, and whether it will work properly is the ability of the banks to provide instant refunds on such transactions.
The information so far available has not answered another possible concern, and that is, when using a credit card or a debit card is, if the system earmarks £99 for the transaction, does that sum get treated as a withdrawal and incur interest on CCs? Or in the case of debit cards, if a customer has funds in an account of £50, but with an overdraft facility of £500. The fuel seller earmarks £99 theoretically taking the account into an overdraft situation, but the actual transaction is only £45. Does the customer incur overdraft charges, even though they have acted sensibly?
Also where there are insufficient funds in an account for the system to earmark £99 for the transaction, it should offer the user the chance to enter a maximum value not exceeding the available funds, and automatically stop the delivery if when the available fund by as been used up.
I don't know the answers, Do you?