Mysterious $1

[fandomfilms]

I'm trying to help a friend with an issue on a site.  When a coupon is used so the shopper gets their product for the shipping costs which in this case is $1.95 they get everything right visually up to the payment.  When they click to pay and go to paypal they are charged a mysterious extra $1.  Does anyone know why this might be?

I'm guessing that since we're sending a $0 payment something on the paypa side can't deal with that???

Need help.

Thank you