if ($_POST['form']['your_checkbox_field'] == 1){ // Where 1 is the value for credit card
     // Script for credit card transaction here...
}else{
     // Script for other payment options go here
}

// If the total is greater than 0 (I.E. if there's something in the cart)
if($_POST['form']['Total'] != 0){
	// Now we've checked to see if there is something in the cart, lets check which method of payment they chose
	if ($_POST['form']['your_radio_field'] == 1){ // Where 1 is the value for credit card
		// Now set up variables to use...
		$business = 'me@mybusiness.com'; // Presumeably this will not change so you could put your email here unless you want it dynamic (multiple sellers)
		$item_name = 'Products'; // Presumeably 
		$currency_code = 'USD';
		$amount = $_POST['form']['Total']; // make sure that you have a field with ID Total for this to work
		$return = 'your_site.com'; // Change this to the page you want them to be returned to from PayPay completion
		header('Location: www.paypal.com/cgi-bin/webscr?cmd=_xclick&business='.$business.'&item_name='.$item_name.'¤cy_code='.$currency_code.'&amount='.$amount.'&return='.$return); 
		exit();
	}else{
    	// Script for other payment options go here - maybe insert some info into a transaction or membership table etc...
		// THIS WON'T INTERFERE WITH YOUR EMAILS/so you can still define some email stuff too.
		// This means that if you simply want the form to process normally if NOT paypal, you could delete the "else{ .... }// End non-credit card script" leavint the last "}// End" in
	} // End non-credit card script
}// End 

// SO LEAVING THE "} //END", ADD THE FOLLOWING...
else{
return false; // I think this will take you back to the form and highlight any errors on the form
}

//initialise a local variable to reference the field more quickly (if you want to
$payment_method = $_POST['form']['your_radio_field'];
 
// now that the payment method field has been initialised as a 'local' var (i.e. in the current page), 
//we can check it's value by displaying it on the screen
echo "$payment_method"; // this will give you the value of the option.
 
// If you're not getting what you expect, try 'trapping' the 'break-point' in the script by 
// placing die(); statements immediately inside your if/else curly brace "{". 
// Add a unique message to let you know which point in the script you got to like:
 
if (1>2){
  die("Of course 1 is not greater than 2");
}else{
  die("1 is less than 2");
}
// You can also place your variables inside die("$statements"); with or without "quotes".