I have been lucky a few times where I will instantly email them with the adjustment i need (same email as the order confirmation), and they would either delay the shipping, or send the extra parts out with free shipping. Doing the live chat if it's available may yield quicker results too. I also spend a good chunk of $$$ with them, so I may get treated differently too...
It sure is a pain in the arse though when you find out too late
EDIT: Just checked my past 6 months order history and have spent a little shy of $2k, so preferential treatment is a good possibility lol.
Edited by brunk - 2/25/14 at 5:15pm