Q:

How to trigger payment flow for one-time payments?

I have a chrome extension that im looking to monetize with subscription and free trial.
I have followed all the guide detailed here: https://developer.chrome.com/webstore/one_time_payments
Everything works, its all good, just now I need to know how to actually trigger the payment flow when a user decides they want to pay for my extension.
I can see that there is a "buy.js" for in-app purchases but im not sure how you are supposed to do it for one-time payments.
The only way I can see of doing it is by opening a new tab to my chrome store page and then somehow educating the user that they need to press the orange button... 

Theres got to be a better way of doing it than that tho, surely??

A:

If you want your extension to be paid using Chrome Web Store Payments, you have to follow Chrome Web Store Payments rules, which include fixed price tiers and the fact that payment must be initiated by Chrome Web Store. The in-app purchases work differently.
So yes, your users will have to subscribe using the orange button in your extension's Chrome Web Store entry. Usually they need not to be "educated" to do that: after all, that is the page they installed your extension from, and the orange button was already there.
Depending on which kind of free trial experience you offer, you can display relevant reminders to your users.
For example, if your free trial limits some functionality of the extension, you can prompt the users to subscribe when they try to use one of the premium functions, and/or display a Subscribe button in a visible part of your extension that links to your Chrome Web Store entry.
If your free trial is time-limited, you can display a counter of how many free-trial days your users have remaining, and the Subscribe button mentioned above. When the trial period is over you can automatically alert the users and open the Chrome Web Store entry of your extension. This latter approach (time limited free trial period) is the one I am currently using in my extensions and so far I've had no problems with the users or confusion on their part.

