Having built a cross-platform app recently as a very small team, I want to put in a plug for RevenueCat (https://www.revenuecat.com/). They abstract away the differences between the Apple/Google APIs so you don't have to worry about dealing with both. You only interface with Revcat. They don't support every last Play Store/App Store API, but I haven't yet found that be an issue. Worth noting that our app uses subscriptions, though, so I don't really know how they do with regular old purchases (I would assume that's supported since it seems to be a simpler case).
Another vouch for RevenueCat, has worked a charm for my project. Saved me a lot of time from not having to learn all the intricacies of both payment system backends.
