The goal, for Tiki5, is to support membership payments via PayPal. So you join a group (ex.: members) and you are sent to PayPal, you pay your dues, and PayPal sends you back a confirmation and 1 year is added to your membership. This will permit to offer paid subscriptions to access restricted sections in a site.
Alternatively, you can use the Dynamic Content System. One thing to watch out for: You may have to remove a line-break from the code provided by PayPal. Because PayPal line breaks are changes to "br" html tags by Tiki parser and it breaks the link to PayPal.