When I want to ship something in the states, I use Holly's General Store in Vanceboro, Me. It's just 10 minutes past McAdam and one of the smallest/easiest border crossings in the province.
All you do is have your package shipped to the store. I usually have it addressed like this:
Jordan Harris
C/O Holly's General Store
47 Water Street (Might now be Railroad Street)
Vanceboro, ME, 04491
1 207-788-3922
They accept hundreds of Canadians' packages a month. I'd give them a call to make sure they still do it, but essentially they just charge you $2 per week holding fees. It's about 50 mins to get there, fill up your gas tank on the other side and your trip is paid for. As for the border guards, just tell them on the US side that you're going to get a package at Holly's. On the way home make sure you have your receipt and tell them you picked up a package. They know what's up.