No extra charge, my buddy there who handles QA is just going to pull them prior to shipping out to me. Then if it works well which I am sure it will they will try to offer that option from the beginning.
Subs are not super common on bikes which explains the lack of a sub amp...until now
Are they going to bridge it for you also. IIRC you have to Bridge it internally ; /
What wattage will you end up with? Any word on a 6 channel?