while I was doing some tests, I've found that paypal.php gateway class have a bug: amount sent to gateway is SubTotal, instead of being TotalAmount:
#51: $ret['amount'] = $this->ConvertCurrency($item_data['SubTotal'], $target);
I've manually checked that this is not the case for all other built-in gateway classes.
And I'm still very happy to see I can't access Mantis in-commerce related tasks... Surely because I can't bring anything good there ;-)