The Web Store Payments API is trash. I waded through the out-of-date, sparse documentation and examples, trawled stack overflow and 'Chromium Extensions' Google Group. Spent weeks of my life trying to find work arounds to various issues. Thinking of setting up a github repo with documentation about all the issues and workarounds. Shame on Google that they leave this online.

In the end, I couldn't get something satisfactory working, and went with Paddle for payments. Cost per transaction is quite high in our case ($0.75 on a $5 monthly subscription) but it was fairly painless to set up, and they handle VAT/sales tax. Would love to find a way to offer $3/month subscriptions without handling tax ourselves.