You might check these two extensions. 1st one is https://community.appinventor.mit.edu/t/free-inapp-billing-google-inapp-purchase-v7-0-0, from @iamwsumit it's free of cost or you want more advanced features then might go with jewel`s InAppBilling extension, https://community.appinventor.mit.edu/t/paid-in-app-billing-advance-billing-extension-v7-0-0