xDonations allows you to 'assign' users to a group once they have donated.
So, and example of making this work for you would be:
Set site up so that registration is required to view information, leaving enough info viewable to non-registered users to convince them to register.
Have certain areas of the site available to registered users that have NOT paid. Adding little notes about what else a registered user can see if they sign up for the service....
Now...the user decides they want access to the stuff they can only see if they pay...they go to your altered donations module, now called 'subscriptions' or something similar, and 'donate' (deposit money into your paypal account). Once their donation has gone through and is recognized by the PayPal IPN, they will be automatically assigned to the 'Paid Members' group that you have created, that has access to the 'paid for' areas of your site.
You may have to check with the author of the xDonations module about altering the submission information to use a PayPal subscription instead of just payments.
**also, on a side note...you will have to manually delete the users subscription from PayPal when they cancel.
My 2 cents worth: The amember software etc. and the set-up required to make it work is perhaps a tad more involved than the above example and requires just as much 'management', as well as costing about $140.
If you are comfortable in XOOPS and or have a decent relationship with any of the many helpful xoopsers out there...you may wish to save your money. Likewise the opposite can be said, you be better off spending the money if you are not at all comfortable with the XOOPS environment.
Happy Xoopsing!