Sorry I didn't spot it earlier. I did a test transaction and then put the error on the failure page. The error from paypal was: "The totals of the cart item amounts do not match order amounts."
So I looked more carefully at the pp_confirm.php page where the ItemTotal is set and found this on lines 138-139:
$WA_PP_ECO_Do_optional[0][$nextIndex] = "ItemTotal";
$WA_PP_ECO_Do_optional[1][$nextIndex] = "".$eCart1->DisplayInfo("TotalPrice") ."";
That is only passing the TotalPrice of the first item to paypal, not the sum of all of the items in the cart.... It should be:
$WA_PP_ECO_Do_optional[0][$nextIndex] = "ItemTotal";
$WA_PP_ECO_Do_optional[1][$nextIndex] = "".$eCart1->TotalColumn("TotalPrice") ."";
I've made that update on your site and I think it should work properly now.