RE: "From what I understand, Visa/MC guarantees the money if it is declined on the tip after the customer leaves."
Well, this is a big gray minefield.
Originally, the card associations did not compensate for tips and the merchant and/or software vendor had to do the over-authorization percentage to make sure the transaction was covered. This is why Aloha and most POS application have some sort of over-authorization setting.
Sometime, I believe in the early 90's, the card associations automatically allowed for up to a 20% tolerance from the initial authorization for restaurant merchants. The actual method to allow for this was left to the issuers and different issuers handled this differently -- same added 20% to the authorization request, others simply compensated for the 20% on the backend settlement side. Either way, it was working fairly well and the merchants did not have to compensate for the 20%.
Then came online banking, the increase in debit card usage and branded gift cards. About two years ago the card associations changed the rules. Now, rate wise, as long as the merchant gets an online approval with swipe data for any amount, the transaction can be settled for any amount -- no more 20% limit to the tolerance. This means that technically, a merchant can now perform $1 and settle the transaction for $10, $100 or any amount and still get the "best rate". The problem is, the new rules do not address the liability and funding side of the equation. With a standard true credit card, this is not much of a problem. But with debit cards and prepaid gift cards, there is no guarantee of any funds in excess of the initial authorization.
As a merchant, you should sit down with your merchant provider to determine the best and most cost effective solution for you. Each situation is different and it boils down to a risk vs. reward decision. While you can't over-authorize the transaction up front, you will need to decide what your tolerance will be before requiring a secondary authorization for tips.
Steve Sommers
-- Creators of $$$ ON THE NET(tm) payment processing services
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