Adding amount to be invoiced to the project customer (Fixed-Price)

Solution is for Fixed-price projects. For Time & Material projects, see this article.

Open your project and choose "On-account transactions" under Bill in the Manage menu:

Choose [+New] and specify Project date, Description and Sales price.
Specify Activity if required by your project configuration. The line will turn "Chargeable" once you hit save.

But dont forget to choose "Financial dimensions" and check if the line has the right "Product" code (initially this is defaulted from Project dimension setup)

Now, you will find these “Fee lines” together with other billable project transactions when creating a new Invoice proposal.
PS: On-account/milestone lines have a project date that needs to fall within the search criteria used.