If you want to charge your affiliates a fee to become approved in your program, then you'll need to use Post Affiliate Pro with some kind of 3rd party membership software, such as "Amember", or have them pay a one time fee with something like a PayPal button for example.
We have an integration method for Amember that automatically registers affiliates into Post Affiliate Pro after they pay the signup fee at your Amember site. Check it out here:
The integration instructions can also be found in your merchant panel under Tools > Integration > Sales/Leads tracking > from the drop-down menu choose the desired integration method.
NOTE: Using this method, the built-in signup form would not be used for signing up affiliates.
Also, in the merchant panel under Configuration > Affiliates signup, make sure to set "Affiliate Approval" to "Automatic Approval."
If you use a PayPal button in your site and you just want to use PayPal to register affiliates, then you can integrate the PayPal button directly with Post Affiliate Pro (use this guide to choose the correct paypal integration) and in the PayPal IPN Handling plugin found in the merchant panel under Configuration > Plugins, turn on the following option: Register new affiliate with every occurred event