function formsubmit() 
{
  if (document.forms[0].invoice.value !='') 	
  {
    document.forms[0].item_name.value= 'Newtons Coaches - ' + document.forms[0].invoice.value;
  }
   if (document.forms[0].amount.value =='' || document.forms[0].amount.value =='0.00')
  {
    alert('Please enter payment amount');
    return false;
  }
  if (document.forms[0].invoice.value=='')
  {
    alert('Please enter an invoice number or payment reference');
    return false;
  }  
  if (!IsNumeric(document.forms[0].amount.value))
  {
    alert('Please enter only numbers in the amount field');
	return false;
  }
}
function IsNumeric(sText)
{
  var ValidChars = "0123456789.";
  var IsNumber=true;
  var Char;
  for (i=0;i<sText.length && IsNumber==true;i++)
      {
      Char = sText.charAt(i);
      if (ValidChars.indexOf(Char) == -1)
         {
         IsNumber = false;
         }
      }
   return IsNumber;
   }