I cross the border numerous times a month, and have had no issue when crossing into the US and "servicing parts" as the reason for crossing.
Like others have said, don't seal the boxes, leave them open.
Save your receipts, as you'll need them to prove work was done on existing parts you already own. I never get charged duty on shipping off parts back to me since it's a service and not "goods."
I don't think they'll charge you duty or tax coming back, as you're only paying for services to be done on your parts.
I'll be doing the same thing in 2 weeks. Sending carbs to harisuluv and a head to MReick.