I am not quite as far North as you, but welcome from North Dakota!
I am glad to hear that your first smoke was a success.
Costco has great Butts, but unfortunately they only have boneless (at least in my area), so I typically go to Sam's Club which has great Bone-in Boston Butt double packs as I prefer the bone-in. So, if you have a Sam's Club, I would recommend going there instead of Costco.
If you do not have a nearby Sam's Club, the boneless butts at Costco are great quality, but you will pay a little more for them (for the deboning) and I would recommend trussing them up around where the bone was to try to make it seem like a one piece unit that will cook evenly.
Sorry, I can't help you on smoking the BBQ sauce. It is an interesting concept. There are several sauce recipes in the Sauce section and would be a great place to post a separate post on this subject.
When you get a chance, introduce yourself in the Introductions section. We enjoy hearing from the new members about how they picked the Smokin-It, Smoking background, what you like to smoke, etc.