Is there any scripts to integrate in WHM for automatic signup process and receive payments with paypal.
Not sure about the PayPal part (though most billing apps seem to support it), but here's a list of billing applications known to work with cPanel/WHM. If anyone knows of any others, feel free to post

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Commercial Applications (Listed Alphabetically)
* Account Lab Plus - Netenberg.com/accountlabplus.php
* Advanced Webhost Billing System (AWBS) - AWBS.com (Formerly: DRAMS)
* Blesta - Blesta.com
* ClientExec - ClientExec.com
* HostAdmin - DreamCost.com/?page=hostadmin (Requires purchase of add-on module to work with cPanel/WHM)
* iHost - iHostDev.com/ihost.html
* iPanel - iHostDev.com/ipanel.html
* Logicbill - Logicbill.com
* ModernBill - ModernBill.com
* PhpHostBot - idevspot.com/PhpHostBot.php
* Platypus - resellers.TuCows.com/backoffice
* Ubersmith - Ubersmith.com
* Whois.Cart - WhoisCart.net
* WHM AutoPilot - WHMAutoPilot.com
* WHM Complete Solution - WHMCS.com
* WHN Billing Professional - WebHostingBilling.com/whnbilling.html
Free Applications (Listed Alphabetically)
* Auto Host Manager (AHM) - AutoHostManager.com
* FastPay - fastpay.EquableHost.com
* phpCOIN - phpCOIN.com (Requires add-on module to work with cPanel/WHM)
* Solid-State - Solid-State.org (Requires add-on module to work with cPanel/WHM)