I offer two different payment plants for large projects.

Staged payments based on milstones with a 40% deposit upfront.

Or fixed monthly payments for the total length of the projects from contract signging to completion.

I would love the option for the client to select in the proposal which payment plan they prefer and then preview what the payments will be in the propsoal.

eg 40% upfronts 40% mid 20% completion

or 6 monthly payments of X