From contact form to paypal

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Maybe not the best coding wise answers, but for quick solutions that I’ve done and are common:

  1. Put the registration page as a thank you from PayPal, which is a
    setting when doing a PayPal payment button.
  2. Put the link to payment in the registration email that’s sent to them, from there they have to pay to become verified.
  3. Use a premade plugin (probably your best bet here) like S2Member (which I would recommend), there’s a lot of free options though. This would handle what I was saying above out of box. This would be the quickest/fastest solution.