Typically Apple and Google will consider coins and credits a type of digital currency. Under their terms and services these types of products in an application must be transacted through the AppStore and Play Store so they get the 30% cut.

When I worked for Groupon, building their Android application, they were not required to use the IAP from either platform. This is because they were selling a product or service. As you stated - you're selling a service. You may need to change the wording from "Credits" to "Balance" (or something similar to that) and have that reflected as actual dollar amounts in the app. I would also be very clear that you are selling a service - not digital goods. If this is truly what you are doing (selling a service), then you should be in the clear.

Disclaimer: These TOS change all the time. The last time I dove in and read one end to end regarding IAP was about nine months ago. But I'm fairly certain the service and product thing still applies. You may want to consult your lawyer and review the TOS with them to be sure you're not in the grey area.