One thing to be careful of is that there is a difference between silver solder and silver BEARING solder.
True silver solder is not sold in wire form on a roll... it is fairly hard and is sold in square sticks like brazing rod. A 35w iron is not going to touch it.
Silver bearing solder is named for the fact that it BEARS solder (it's not designed for soldering bearings). It can be as little as 2-3% silver blended in with conventional solder.

Dunno about the stuff that Radio Shack is selling... I have lost a lot of faith in them over the years... their shrink tubing is flat out junk.