low coolant after replacing radiator, tank, cap and hoses??

[vshinclt]

Hello-
I would greatly appreciate your help/advice re: my 2002 S80, 90K miles. 9 months ago I was told I had a radiator leak and the mechanic patched it for me. Before this, my driver's side floorboard was sticky/wet. The patch seemed to hold fine but I didn't want to tempt fate with hot NC summers. About 3 months ago, at the start of summer, I bit the bullet and paid a shop that specializes in radiators to replace the radiator. Cost was about $800. 2 months later, the light came on for low coolant (the clear tank was low but not bone dry and there was some crusty stuff around the top of tank). I added water and took it in again. Also around this time I noticed a "musty" smell coming from A/C vents. They replaced the clear plastic tank, some hose clamps to the tank, and the pressurized cap to the tank. Cost was about $200. The next day after I retrieved the car, the low coolant light came on. I drove home (less than a mile) and checked coolant tank -- it was completely empty. I had driven the car less than 20 miles total, no highway speeds, since picking it up from the shop.

As you can tell from my language, I know NOTHING about cars. After 2 trips to this same radiator shop and continued problems, I am wary of the quality of their work, but also inclined to go back as they gave me a year's warranty on their work. Anybody have any thoughts about what my problem might be? Are there things I could be aware of that might help me know if this shop is reputable? Thanks so much in advance.
Virginia