title: service.acknowledge is undefined using Digital Goods API with Google Play in TWA
body: <p>I have an web app in development to which I am trying to add Google Play Billing on Android and Chrome OS.  My implementation is basically the same as in <a href="https://developer.chrome.com/docs/android/trusted-web-activity/receive-payments-play-billing/" rel="nofollow noreferrer">Google's own documentation</a>, however it fails at the point of acknowledging the purchase after it completes, throwing <code>TypeError: service.acknowledge is not a function</code>.  Everything else works fine, but not being able to acknowledge the purchase means Google Play automatically reverses it after a period of time.</p>