When you buy mussels by the bag (lbs), what proportion do you expect to be live vs possibly dead? I do the tapping/thumping test after rinsing and pulling the noticeable beards and put aside the ones that will not close (almost all of the way). If I notice slight movement (closing of the shell) but they don't fully close, I am ok cooking those. Assuming proper storage, I assume I will have a few dead ones but not many.
I ask as we recently bought one of the net bags (5lbs I think) from Costco as we were leaving then headed straight home on a cold day. I believe we put them in our cooler bag but otherwise did not do much with them until we got home (15 minute drive) and put the bag in the cold fridge. We washed them a few hours later before preparing and a lot of them would not close or the shell was damaged enough to conclude they were dead. I bagged the "bad" ones and weighed the bag which came out to 1 lb, 1 & 5/8 ounces. Obvisously as it is Costco and I froze the dead ones, I can get my money back (although I am not sure I feel the need to get 100% back). However, had I needed close to the full amount for a dinner party or something, I would have been very iritated as opposed to mildly annnoyed.
When prepared, the ones that were not dead were delicious.
Anyway, what do you expect?