We need to take a closer look at the checkout validation routine. I had 4 orders get through this weekend with no carrier/ship method selected, and no value for the shipping amount. I am using shipping by quantity ordered.
Since we're doing a pre-authorization on the sale, I can't add the shipping amount (and I have had customers say that they selected "UPS 2nd day" and it came over as blank). Anyway, I can't add the shipping amount, because I can't settle for more than I authorized. So, this will be four more orders that I have to eat the shipping on.
Now, I can't regularly get it to replicate this problem, but it obviously keeps happening.