How do you deal with the risk of customers charging back their visas? Simple, you don't. You just increase the margin on the products to account for occurrence of the fraud.

By waiting less than 10 seconds, you will be able to validate a 0 confirm transaction doesn't have an existing double spend, so then you only have to worry about a Finney attack, which will be so rare that the money you save in not paying a credit card processor will more than make up for it.