I tried to send Model 28 into the shop three years ago. S&W said it was to old to work on... so I sold it off.
When I bought my FS M&P45, I was under the impression that it was a DA only pistol, similar to the trigger on my DAO 4586. Wow, was I ever disappointed. Last month I asked S&W if they could install a thumb safety for me. I don't care for the combination of a light trigger without a safety. The hinged plastic trigger may satisfy lawyers, but not me.
Adding safetys to guns that came without safetys is not a service we provide here at the factory. If your experiencing a defect with your trigger i would recommend to have the gun sent in to be evaluated."
I'll probably end up selling the M&P45 too.