One well known trick in the orthopedic community for shoulder pain, when using a mouse, is to lower the mousing surface into/onto your lap. This will make a huge difference in not having to forward elevate your shoulder and take the stress off your rotator cuff and associated irritated tissues.

Also as for "carpal tunnel", this has always been a hypothesis as to being caused by computing, but good experimental research has finally put this idea to rest (as in not supported as causative), but the belief is so firmly embedded in the culture now that it is hard to extinguish and manufacturers continue to place warnings on their products to avoid liability. Interesting intersection between yet to be proven initial hypotheses, experimental medical research, modern culture and the legal system.