Congratulate them
Mark the finish with a warm note.
List what you are handing over
Name files, logins, source files and documents.
Check final payment
Say what is due before the handover, if your terms require it.
Explain how to access everything
Give links or a secure way to receive files.
Say what support remains
State any warranty period or how to ask for fixes.
Ask for a review
Invite a testimonial once they are happy.