Upfront Costs are all costs not paid per-second. The distinction is necessary because Link skills introduce the concept of per-second costs to PoE, and some things can’t work with those so now need to specify.
This is not a functional change, just needing to be more specific since there’s now a new type of cost for which the amount spent can’t be tracked on a per-skill basis.
Each time you pay the cost of a channeling skill is paying an upfront cost. But you are incorrect about channeling being rapidly casting – it’s one long cast, over which you pay the cost multiple times.