Ideas/Suggestions for Discount Coupon - How to give a credit and a discount

I have a client that wants to do the following:

She gives her staff a monthly credit for the store as their pay. She would like to let them order what they want up to the certain amount (the credit) and then anything above that, they get 20% off.

Does anyone have any good ideas of an easy way to accomplish this goal? I can create a coupon for a $20 credit, and I can create one for 20% off, but I don't think you can use multiple coupons. I thought about a gift certificate, but that would be messy.

I have thought about this for a few days and thought I would ask the brain trust on here. Maybe it's right in front of me and I am just not thinking about it, or maybe it's not that easy after all.

Any tips/suggestions/ideas are greatly appreciated, thank you!

Mike