Re: Require Payment to Join
... This require deep thinking (no approximation when $ is involved).
At first I would say that the payment validation is a manual operation.
You check paypal and validate manually the confirmation money is arrived in your account.
Once this is done, you have many way to "upgrade" or "create" a user within a specific group. From registration page with keypass (easiest dumb) solution to filtering with a tracker list. Some javascript can be use if you want complicate, but it is important to keep in mind that the process start with a manual validation.
At least for me !
Form experience, I would trust a validation based ONLY on Paypal script response without checking the money arrived (no cancellation, usage of cards with limits - like rechargeable, etc).
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