A coupon offered to my clients resutls in $0.99 processing fee

I'm charging $12 for a 5x7. My corporate client payed me to give free 5x7s to 450 people. I created a coupon for them for $12 + $1.99 for shipping.
Because after the coupon is used, the resulting cost is $0, Fotomoto charges a process fee of $0.99! I have 2000 people visiting the web gallery and purchasing prints and this puts me at risk of having a problem with my client. Please advise
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