Basic answer, if you re-rate your film for better shadow detail you DO NOT tell the lab to do anything different. What you are doing is adjusting your exposure to the entire chain, which includes development.(posted 8602 days ago)I agree that unless you are using C-41 processed B&W film (Kodak TCN400, B&W Select+ or Ilford XP2+) you will not be getting optimum results.
Developing film yourself at home is not hard, nor does it take a lot of equipment. You don't need a darkroom, just a dark place to load the film onto the reels and into the tank. This can be a changing bag, a closet, a bathroom, or any place where you can get it really dark.
Check some of the other threads about this.