Do you mean you want them to pay once, for all courses on your site? Paypal is an enrolment method for individual courses, not an overall authentication method for the site so I think the only way you could do this is by adding Paypal to one general/empty course, which you could link to on the front page - perhaps with a graphic in a topic section? Then when they have paid to enrol in that course you can use their info to manually enrol them into other courses.
General help
moodle 2.8
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