Whether you wire them yourself, or have a licensed contractor do it, it's all paid for by the customer anyway, and no place to "cheap out" or "wing it". Commercial buildings have strict codes, and get periodic inspections. Violating codes can be very costly if you're caught, and having a failure that results in property damage, injury or death by electrocution or a fire if anything fails is the kind of liability that you don't want. If you're unsure how to do it within code, make your money off the sign & install, sub an electrician to do the feed, bill the customer, and move on to the next job.
Anything through walls must be rigid conduit in commercial buildings, period. If there's an existing external service point, and you have to use flexible to attach a fixture or sign, for whatever reason, wires should be in sheathed, liquid resistant, exterior rated, flexible conduit, not just flexible wire. Adds a few bucks, but you won't have an inspector pull out their citation book if/ when they do an inspection. Conduit also isn't affected by sun and weather damage that can cause failure, can't be chewed through by critters and start a fire that you'll be liable for... That's why they have these codes.
