Well, you’re probably in the right subforum. Automations and the Scripting block aren’t your only options though. But for clarity, the former would run sporadically to check for new customers, while the latter you’d run yourself by clicking a run script button every now and then.
Assuming you’re willing to think outside of the checkbox, though, you should be able to get this behavior with formula fields alone, since the pattern sounds identical for every customer, with the only variable sans their identities being the date on which they get billed. Why check a box after inserting the payment details if a formula field can do it for you? Why do the paperwork by hand, at all? Assuming I understood you correctly, that is.
