Mobile strategy that works. Build and ship faster while growing and continually improving your mobile presence. Real world experience with top apps. Software Consultant (main focus is on Web and Mobile). Advisor for Picmonic. Best Selling Android Author. Overall 4 Time book author. National Speaker. Vast experience with a focus on helping solve mobile and scaling issues for startups at various levels. I have two apps currently in the Top Free category on Google Play. Developer of the Groupon Android App, MyFitnessPal Android App, Eventbrite Android app (V1) and many more. Open source contributor, founder of AndroidJobs.IO and AndroidBootstrap.com. Helped build the worlds first smart payment terminal - Poynt. Currently helping Realm implement a better database for mobile. I can help you identify the proper direction when developing your mobile strategy (for both platforms). I can assist with advisement in regards to proper API design for mobile consumers. The one question I help many new startups answer is whether or not they should use a cross platform solution or go native. I can help you answer that question, quickly.
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.