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i've never seen a subscriptions module for xoops, but you could take my xDonations module, and modify it
that's a future feature that we'd like to implement, but we need to get Groups supported first.
http://dev.xoops.org << you'll find an easy link to xDonations on the right. i suggest trying the CVS release, it has a needed bugfix, in addition to a couple of nicer features.