Wow, good question. There are no firm rules about the use here, except I will tell you in general the way they are used. Both can be used and understood to be the same meaning. Packs usually refers to larger numbers of an item, whereas sets are usually used for a limited, smaller amount of an item,. As you said, 48 paper plates come in packs, and maybe a nice tea set of 4 cups will come as a set. Sets tend to be used in more defined limited numbers of the items, where packs can be used for larger amounts of items. As far as the coffee, there is no right or wrong way, but it sounds more natural to say you have a 10 pack of coffee.