It's hard to say what Tire Discounters did. Almost universally, the standard option is to replace the whole strut kit. It's super easy labor and they pass the cost savings on to you. Shop rates being what they are, it rarely makes financial sense to pay someone to uninstall a strut, disassemble it completely, replace a single bad part, then put it all back together. Yes, the strut was warrantied, but, if you have to pay for the labor for the mount, it may be cheaper to just replace the whole strut kit.