<form name="FormRecuring" action="https://www.paypal.com/cgi-bin/webscr" method="post">
...
<script type="text/javascript">
	// Validate dollar amount upon clicking Submit2.
	// if validation is successful, perform a 'submit' on the form.
	function SubmitRecuring()
	{
		// See if there's a leading '$'.
		if ( FormRecuring.a3.value[0]=="$" )
		{
			var iLen = FormRecuring.a3.value.length;
			// Strip the leading char.
			FormRecuring.a3.value = String(FormRecuring.a3.value).substring(1);
		}
		
		var DollarAmount = parseFloat(FormRecuring.a3.value); // Convert to a float.
		DollarAmount = DollarAmount.toFixed(2); // keep 2 digits to the right of the decimal.
		// alert( DollarAmount );
		
		if ( DollarAmount <= 0 || isNaN(DollarAmount) )
		{   
			alert( "Enter a dollar amount" );
			return false; // return without submitting the form.
		}
		FormRecuring.submit(); // Submit the form.
		return true;
	}
</script>
<!-- Comment-out this input control; replace 'input' with 'img' and add 'onclick=' to call SubmitRecuring().
<!-- input ilo-full-src="http://images.paypal.com/images/x-click-but31.gif" name="submit" src="giving00_files/x-click-but31.gif" alt="Make payments with PayPal, it's fast, free, and secure!" border="0" type="image" width="72" height="32"-->
<img onclick="SubmitRecuring()" ilo-full-src="http://images.paypal.com/images/x-click-but31.gif" name="Submit2" src="giving00_files/x-click-but31.gif" alt="Make payments with PayPal, it's fast, free, and secure!" border="0" type="image" width="72" height="32"> 
</form>

This demonstrates how to validate parameters prior to submitting a PayPal form

PayPal parameter validation

JavaScript