The hourly charge is the most significant factor. Even lower hourly rates are possible by taking out a monthly package, which gives you a certain number of hours 'free' by buying up front. Most ISPs bill in 5 minutely increments. Volume charges refer to how much you pay for every page of information you download. No large providers have such a cost.
Some smaller ones do, and they end up being exorbitantly expensive.
Package refers to the overall package that is available - do they have special packages, do they provide proper user support, is the software registered and properly configured etc. See the list below. The things most service providers will not tell you up front. Return to Outline Modems and Serial Cards Faster is better - less frustrating, more efficient, cheaper. Currently 28K modem is mandatory.