Mussels - proportion of live to possibly/probably dead
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?
First of all, I'm very surprised that you had such a poor experience with Costco's mussels. We buy those 5# bags all the time from our Costco here (they're tagged from Prince Edward Island), & have spoiled us re: buying mussels from any other vendor. They're THAT good. Last bag we purchased there last month didn't have one single mussel that was damaged, wouldn't close, or didn't open during cooking. Not one.
That said, prior to buying our mussels from Costco, out of the usual markets' 2# bags, I always expected to discard no more than 3-6 mussels tops. More than that, & I simply returned to the vendor with the tag & the mesh bag that the mussels came in & requested at least a partial refund. Was never asked to bring back the dead/damaged mussels. Any vendor who wants your continued business won't expect you to (unless you're a constant complainer, & I'd imagine if you were that displeased, you wouldn't be a repeat customer).
Over a pound of damaged/dead mussels is way too much to have to toss - even out of a 5# bag. I'd expect a full refund.
I once had a fishmonger that would tap on each one to ensure they were acceptable. Everybody behind me in line were very patient, as they expected the same level of service. He encountered about 10% from the wholesale bag of mussels. He did not seem surprised.
Wish he was still around.