If you have an add-on you always want to show on the "Check Out" list, we have a tool for that.
- Create your addon.
- Go To programmers settings and at the end you will see Permanent Addon.
- Turn it ON and you will see a text box asking for permanent addon ID. See https://tinyurl.com/ydbgj9aw
- You can go to Edit page of addon, and in URL you will see its ID. See https://tinyurl.com/y7wh2sea
- After you get ID, in site settings Permanent addon ID textbox, just paste/enter that ID and save.
- Now this Addon will become a permanent Addon. And you will always see it in “Checkout popup” in admin section.