Really great suggestion, definitely something we could do in the future. We just rolled that service fee out this week, so we're watching it closely. Right now, we offer 5 free purchases regardless of how the user pays, but something that's already clear is that we'll need to remove it from the stored balance reload.

As we add payment options like ACH, PayPal, etc. we'll have more wiggle room to adjust the fee based on the final payment method, and establishing a running total like you suggest could increase that variability even more.