It's partly to lessen the fees being paid for processing each charge, but also part convenience for the user. We actually implemented a prototype top-up system to see how the user experience would be, and everyone who tried it agreed it was fairly cumbersome so we quickly scrapped that idea. And bundling the payments makes for a cleaner card statement, although that aspect is fairly individual.

As for the volume we would have to do; Stripe for instance charges 30 cents plus 2.9% for each charge, so the first article a user buys per month pretty much covers the flat fee, after that the percentage acts the same regardless of volume.

Great feedback though! Keep up with the questions.