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This addon can't fill pre-existing custom fields. This feature is on the wish-list but not likely to be added soon I believe.
One can set redirect urls in ACP for each form, so you can link to PayPal url if you like. But usually the common expectation is that it redirects you back after payment, but this addon can't do that (I assume, I never used payment with XF).
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Possible, see 5.1.11:
https://snogssite.com/threads/advanced-forms-documentation.3677/#message-anchor-5111
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Also possible, see 5.3.4:
https://snogssite.com/threads/advanced-forms-documentation.3677/#message-anchor-534
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Not possible. XF doesn't have that build-in, nor this addon.
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On this site you can see the basic working of it:
https://snogssite.com/form/
But there is no demo for the full thing, where you can test the Admin Control Panel options and set your own stuff. You need to buy if you want to see the whole thing. 30$ for a lifetime license for such a complex addon is really a bargain, can speak from experience. Or just read the documentation:
https://snogssite.com/threads/advanced-forms-documentation.3677/ , it lists 95% of everything it has. The left out 5% is because after I wrote the documentation, some new stuff was added but I didn't have the time to update the documentation yet.
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That should be fairly do-able with XF. Just set up usergroups. Then use the user upgrade system of XF when one gets promoted to a usergroup after payment. But how you handle the data is of course half possible and half not possible with this addon.