Another thing to do when you get home is to measure your chain yourself to see if it really needs to be replaced. Bike chains are exactly 1/2" long per link, so if you hold a ruler up to one the pins should all line up nicely on the 1" and 1/2" marks. The way to tell if a chain has become too worn is to line up one pin with the end of a ruler and check to see that another pin about 12" away also lines up right at the inch mark. If it's off by 1/16" or more then it's time to replace the chain.

By the time the chain wears enough so that the measurement over 12" is off by over 1/8" then there may well have been enough wear on the cassette so that will need to be replaced too. But I would always first try just replacing the chain and seeing if there's any skipping before changing the cassette.