So there's a lot of people having issues with their topups, here's how to do it using an Android device and your google play account.
First, you want to add a payment method on your google account. You can do that here: https://wallet.google.com
Once you have added a valid payment method, make sure you have that google account linked to your device. After doing so, you can proceed to open the app and purchase your bellies.
Once we find our selves in the belly selection window, we just have to choose our belly quantity, once we have done so, Google will guide us through the payment process. I recommend closing the game once the successful payment message has been displayed, to make sure no data is lost.