You must be logged in before you pay. Your account gets automatically updated.

Weekly downloads is the number of pages you check every week. E.g. 10 pages 7 times a week is 70 weekly downloads (free).
20 pages two times a day every week day is 20x2x5 = 200 weekly downloads, which require account level 2 (10 UDS a year). A Priority account means you will be put in front of the line when the server calculates differences, and your updates will be delivered at the time you specify.