Incorrect SUM if non-calculated field contains multiple values

I've created a point of sale system for a small salon and I'm using views calc for building client history information. I've created a views block containing their transaction history with a sum of all transactions. I also have a multi-value field containing any payment methods used; cash, check, and credit. Unfortunately, since implementing the payment methods field, all transactions are totaled 3 times, one for each payment method. Under the payment method field settings, I have the payment methods grouped, but views calc still treats each as a separate item. Is there any way to prevent views calc from counting these separately? Any help you could offer would be appreciated.