My thinking with any painter decent communication is key and even then things can go haywire.

My thinking:
Photo-document the existing well, and apply measurements to key transfer placements to those images.
Do the transaction face to face if you can.
If shipping, think about how someone could repack the frame in the same box and packing effectively.
(I have done this, only 50/50 chance, but when it was not done the frame got damaged on the return)
On color... I am very particular. I trust Brian Baylis, but he is not seeking work, everyone else I will request and pay for a sprayout with me furnishing a section of tubing. Or, find out what paint system they are using an I buy the paint. Sounds silly but actually works pretty well. "trust but verify" to paraphrase Ronny Raygun. Ask for a splotch of lug accent color too if applicable.
Find out ahead of time how threadings are protected or not. Do you have that 3mm tap for dropouts and or access to a set of bottom bracket tapes? If you are using a cartridge BB instead of a traditional assembly, advise as some mask the BB shell faces. I provide sacrificial 5 mm fasteners for bottle boss and shift boss fittings. Same for the seat lug if so equipped.
If the BB shell is pierced, discuss how the inside of the shell can be coated, even only the epoxy sealer is applied, this provides peace of mind going forward.
On gloss and paint thickness if you have preferences discuss them upfront and give that guidance in writing, do not depend on the artist to remember stuff, your frame will be one of many, while you just get to focus on one.
Note chrome masking, style and placement.
Know if the frame, fork and ends are straight before you take it in. This avoids the hidden up charges of some and if things need correction it is done BEFORE the new finish.

Some of what I have advised will cost more, but painting a bike can get quite expensive and the possible extra 10-15% you spend will help avoid lots of dismay later when it is done and not quite right. No one wants to ask for a redo, and the painter will not be that excited either. Sometimes transfers are scarce and there may not be a second chance.

I also think there should be no surprises with the final bill. None. Anything else is just not sound business. If they can't come up with a not to exceed estimate that you can live with be warned, you will be unpleasantly surprised. I have been told that some who do wet paint sometimes use powder coat for the base layer, NOT acceptable to me.