movie charged by Xfinity instead of Amazon

I visit my stepfather (92 yr) frequently to help with various things and sometimes we watch movies on his Xfinity TV setup. I have an Amazon Prime account so we bring up the app, and I want him to get used to that when he's searching for something specific that would incur additional cost, for example Thursday night football. Recently I noticed that if we use the Xfinity voice search (i.e. "listen" button on the controller) then the movie is *still* charged by Xfinity, but when I used the manual search (i.e. letter-by-letter) than I saw the expected Amazon charge. Am I seeing things or does this actually happen ?  If so what can I do about this ? It's unreasonable to ask my stepfather to be doing manual letter-by-letter searches with his controller when I'm not there.

Thanks