When you are using Payments Standard one of the biggest limitations is knowing the status of the transaction when it occurs. The problem is that PayPal does not process the transaction immediately then send the user to either a failed or success page. When the user submits their payment the status of the transaction is unknown. PayPal only sends the user back to the return page that you specify. You cannot assume that the transaction was completed successfully just because the user gets to this page.
To confirm the transaction you need an IPN page. This page is a page that is never visited directly. It just sits in your site and waits for a response from PayPal. Sometime after the transaction has occurred PayPal will send the information along to this IPN page. It is up to you to set this page up to listed for the response and do what you would need to with the information given. Usually you would record the status in your db and then send out an email receipt to the customer.
I think that using Payments Standard with an IPN page is the most difficult way to work with PayPal payments and provides the most inconvenient shopping experience for the user compared to the other checkouts that PayPal offers. If it is at all possible I would suggest that your client go with a Payments Pro account and implement that checkout with eCart.
Here is a link to more info on IPN from PayPal:
ipn
