I picked up 5lbs of mussels today for dinner and didn’t realize until I got home that they never put them on ice for transport. It’s was about 1.5 hours between purchase and the time I put them in my fridge. They were in an air conditioned vehicle the whole time except for about 20 minutes when I ran into the grocery store. The car did heat up then.   When I got home and realized there was no ice I called the market for advise and they said toss them, they probably died. In an attempt to avoid driving almost an hour to get more I dropped them in water to see if they floated or sunk and I saw a number of them swimming and moving. I assumed they weren’t dead at that point and went to wash each individually to inspect them. The vast majority were tightly closed, with only about 20 mussels of the 5 lbs slightly open. There was also one middle that was wide open but snapped shut when i put water on it. (Again, I’m assuming that’s definitely alive.)  After finding the vast majority still tightly closed, I called the market back and they were very unhelpful. The manager said she sticks by her original statement that they are probably dead, but if they were tightly closed they would probably be ok.   So, I need advise, I don’t want to make our dinner guests sick, but I’m also hesitant to make the 1 hour drive (one way) to pick up more mussels. Should I just chalk it up to a loss and do a different dinner? Or am I totally safe to serve them????  
The mussels are most likely fine.  If they are visibly alive, it means that they spent the drive in an environment which was not harmful enough for them to die.  Even if some of them died during the drive it must have taken them some time to die. They spent the rest of the time in an air-conditioned car. Even if they died very soon after departure, they only spent an hour or so in conditions which were still okay for some of them to survive.   Do not listen to roetnig's advice about closed mussels. It is a well-known myth that they are necessarily bad. Wikipedia article on Mussels says:  Mussel shells usually open when cooked, revealing the cooked soft parts. Historically, it has been believed that after cooking all the mussels should have opened and those that have not are not safe to eat and should be discarded. However, according to marine biologist Nick Ruello, this advice may have arisen from an old, poorly researched cookbook's advice, which has now become an assumed truism for all shellfish. Ruello found 11.5% of all mussels failed to open during cooking, but when forced open, 100% were "both adequately cooked and safe to eat."  The article linked on Wikipedia is not a scientific paper though. I hope someone will do a more thorough analysis of closed mussels and their safety, but the popular rule of thumb seems to be pretty false. 