Setting up a store with credits *and* real money?

MarketPress has support for credits now, but I don't know how to use them in conjunction with an ordinary currency - I can only seem to select 1 type of currency for the entire store.

My use case:
- Customers can book appointments by paying either $100 or 1 credit.
- Customers can buy 5 credits for $400.

Hope some of you can help. =)

I am open to workarounds, such as having 2 appointment products or only having 1 that costs a credit, which can then be bought for $100. Ideally, I'll want a smooth and intuitive transaction flow for my customers though.