I think I do see it, and I honestly thought maybe there was another plugin that'd do what you want without need for customization, that I'd be doing you a disservice making you purchase s2Member Pro if there was something that'd be easier to adjust to what you need. Now, if you've searched and decided s2Member is for you, I'll be happy to help you figure out how to adjust it to your model.
OK, so you want to have members and sell them pages of your script reading service. I see the users may have some credit they bought and should be able to add more. You could let them register a free account to which they'd add the page credits with their purchases.
For the page credits you could have a custom registration field. It should be set so they can't edit it or they'd be able to add pages they didn't pay for. The problem is that there isn't a visibility/edits setting that'd work well in this case, you need one that doesn't show during registration and is not editable afterwards.
WP Admin -> s2Member -> General Options -> Custom Registration Fields/Options -> New Custom Field -> Allow Profile EditsAnother option would be to save it as a new field in the usermeta table, and you'd provide your own interface to display and edit it. The display could be somewhere in the theme, maybe the header, or edit the profile page to have it there.
You will need a custom script to add the new purchased credit to the user's field. The credits could be sold with the pro-forms. You need a way to change the quantity/amount and have that come back to s2Member after checkout.
In the pro-form's page you could have another smaller form with a quantity field, which would call the same page on submission and this quantity value would dynamically change the pro-form's shortcode attribute for the amount.
viewtopic.php?f=36&t=1604After payment you could use the Notification API to fire your script that'll add the new credits to the user's account. Knowing how much a single page costs, from the amount paid you can tell how many pages he bought.
WP Admin -> s2Member -> API /NotificationsAs you can see, it needs some customization, but it can be done.
Does that help?