Good idea. Before you get him to draft the letter though, you should try to negotiate a flat fee for the service. You don't want some guy that bills $x00/hr spending a few days on the letter.
The letter should be an hour discussion plus an hour of drafting, at the most. @vellum is right, always negotiate fees and estimated level of effort up front. In these types of cases it's generally best to deal with each task discretely until, in a worst case, you must consider litigation. This helps avoid locking yourself into a significant retainer when a single letter might do the trick.