How can I add a checkbox to my Pro-Form?
How DO you add checkbox fields to the Stripe form template so that they will be saved to the database?
s2Member allows you to create and manipulate Custom Registration/Profile Fields in various ways. Among the different kinds of Fields that you can create, is the ability to add a Single Checkbox, or a set of multi-option Checkbox Options. See screenshot below.
Referencing: WordPress Dashboard → s2Member → General Options → Registration/Profile Fields & Options