if you dont want to incent them to work as maintainers, make it clear the project is one off, and calibrate a portion of the pay on how maintainable the code is.
if you assess maintainability when you see the code, make what you pay them depend on how maintainable you assess it.
if you assess maintainability over the lifetime of the code pay the freelancer a small retainer over that lifetime based on how few problems maintaining it there were.
if you assess maintainability when you see the code, make what you pay them depend on how maintainable you assess it.
if you assess maintainability over the lifetime of the code pay the freelancer a small retainer over that lifetime based on how few problems maintaining it there were.