The mussels will arrive in damp newspaper, inspect them to make sure they have travelled well and their shells are intact and then simply place them gently into a deep tank, out of direct sunlight, helping to keep the water cool.
Mussels are filter feeders and ideally will not want to be stored in clean water, they would normally bury themselves in the mud at the bottom of a pond. They feed off plankton, pond debris and other microscopic organisms, by drawing water in through an incurrent syphon and expelling cleaned liquid through an excurrent syphon.
Put them in a deep vat, if possible with free-flowing water and maybe even some oxygenating bunches and fish. Not too many together, as they do like their own space, but short term, more crowded than the 1m³ they should have in a pond is not a problem.
Please note that all such mussels are sensitive to aquatic medications, in particular those which contain copper.