I asked this question over at stackoverflow too: Link.
I guess I could have more luck here. I don't want to just copy my whole question from stackoverflow, so please read it over there, if you need more details. But my question in TL;DR format:
I have an Alexa-Skill and am building a Web-App for this skill. In the skill users can buy a subscription for an ISP and in the Web-App I need to know if a user is subscribed (is entitled) to that ISP. How can I do that and is it even possible?