What I do in proposals (I repair pipe organs) is include a note _in
the written proposal_ to the effect that "Change orders, if needed,
will be requested in writing, to be authorized by returning a copy
with initials of a responsible person (the customer if an individual)."

That way, if changes need to be made there is a written record.  It's
also conceivable in my case -- sometimes during major organ work --
that the church might request something additional during the course
of work.

Rick Hunter