If you use eCart you can use Express Checkout and do the transaction directly from the web site without needing to wait for a reply from Paypal.
If you use the free toolkit paypal buttons, then the only solution is to use IPN, which would probably require a premier support ticket to help you build and configure since IPN can be difficult to debug and you have to pass extra information in the created form using a hidden field named "custom" to keep track of important identifying information about the purchase.