The charging isn't done on the client, it happens later on the server. All this checkout does is turn a user's credit card into a token that you can safely send

The charging isn't done on the client, it happens later on the server. All this checkout does is turn a user's credit card into a token that you can safely send to your server. Then, server side, you can charge the card or sign the user up for a subscription. So, in other words the price is client-side, and only used for display. The token gets submitted in a form to your server. And you build the receipt page.