In the browser, you have three options:
An “all you can listen subscription”:

Buy with ticket from what I understand to be a “ticket subscription”. This is what I used, because it seemed like the cheapest option. I think I bought a ticket subscription that gives me one ticket a month (by using this option), used it for this audiobook, and then immediately cancelled that subscription afterwards:

Buy normally (after clicking on プランを選択せずに作品を購入する):

There also seems to be a concept of “points”, but I didn’t get to use them. Maybe because I used the “ticket subscription” option, which was cheaper by default…