Add contacts upon Paypal purchase – allows you to add contacts who purchase your products to the list of contacts. The opt-in settings are defined by your current web subscription settings (campaign settings>>Permission tab); this can be used only with BUY NOW buttons. Integration adds contacts to the cycle right away.

To add PayPal Buy now buttons to your messages or landing page follow the steps explained here (this is a 2nd integration).

Do not combine those two integrations.