This is very relatable, whenever someone asks for anything of any size then they’ll never know what they want until you show it to them.
I’ve started to do a bit of overengineering every now and then when I have a hunch they might want to charge something about it later.
I created a GUI for changing every single string of text on a webapp for admins, showed it to them 10x and they complimented me on how easy it is to use and change any small string in it.
Project then gets halted for Corp approval reasons and they come back with a PDF of changes they want where half of it is text changes.
I recommend insulating yourself from stuff that is subject to change like payment providers or other third party integrations. In ecommerce everyone wants something like “shipping but different if the client’s name ends with ‘SHIP1’ but use default if his number is the default number” and since they asked for the wrong thing you’ll have to do a fast revert.
Never mind, this is getting into rant territory, lol.