A brief update ... your site is using URL masking, which is no longer compatible with PayPal's website-payments-standard product. The PayPal IPN interface checks the URL in the browser, and if it's not a paypal address, it blocks the transaction in order to prevent spoofing and fraud.

Turning off URL masking on your site should resolve the matter, at least according to PayPal tech support's suggestion after looking at your site.

Naturally, you could retain your URL masking if you chose to use PayPal Express Checkout instead of PayPal IPN module.