I have a separate (Firebase) project for each customer. I want to let each customer pay for the project I created for them. I don’t want the customer to be able to see anything about the implementation of the project (no access to the Firebase console in my case). How can I go about this?

I found this question that suggests some options, but the preferred solution of letting the customer be the owner of the project didn’t satisfy my needs of not giving them access to the implementation.

At the moment I’m manually creating invoices for each customer separately, which takes time. I want the costs to be paid by the customers themselves instead of first by me and then by them.