Corbin does the custom stitching on their own seats. It is not any extra if you are getting one with heat, but is extra if your not I believe. JCZ posted some of this information in his thread. If it is not to late you should be able to call Corbin and have your order amended. Some people have had trouble getting the exact color that they wanted, because Corbin has many thread color options, but doesn't send out samples of the thread colors.
My recommendation if your order is not complete is to order your seat through HD-dude. That is what I would have done until he offered up the same seat I would have ordered early and pre-made.
Ordering through Jim will get you a 10% discount because your a member of this site as well as great customer service from someone who knows already how to interface with Corbin.
Just my 2 cents.