Hi, Pipper,
Legally, anyone can send a gun to any FFL holder, which includes manufacturers and warranty repair centers. As far as I know, FedEx is still accepting handgun shipments. Many people, including myself, prefer to ship through a dealer to avoid any hassle with the carrier, and willingly pay a small fee for the service. I am on good terms with the area dealers and they will usually ship for free if I package the item and pay for the shipping.
I suggest you write to S&W, tell them what the problem is or what you want done, and follow their instructions.
Jim