My shower handle became stiff, then failed to turn on the water. A plumber visited and, without opening anything up, announced that he would have to do a repair from behind the pipes, gaining access through the back of an adjacent closet. He was also going to replace the tap/diverter since the diverter had become stiff. His charge was $1180 + tax but, without any prodding, he "generously" reduced the price by $35 (he called that a coupon) and a 10% senior's discount. Friends suggested that this sounded like a very inflated price. When I asked the plumber for a detailed breakdown of his charges and the names of the parts he planned to use, he cancelled the job and wished me luck.
A friend suggested that the problem was the cartridge. I researched the repair for my shower type (Riobel) and found that Centennial Plumbing Supply was a Riobel distributor. The men who assisted me during two visits (Mike and Steve?) were extremely patient and helpful. Thanks to them, I just repaired my shower for the cost of a cartridge and some CLR (that's all it took to mobilize the diverter). The evidence for how helpful they were is that I'm a 67-year-old woman who has never done any plumbing.