ALWAYS call local US Customs at your port of entry prior to departing Canada when re-entering the US. EAPIS is not enough.
If crossing the border on a VFR flight plan, get a squawk code from either Nav Canada or Canadian ARTCC when airborne for the crossing. 1200 is OK going into Canada, but not for crossing back to the lower 48. They don't care about a code crossing into Alaska, just the Lower 48.
I would also outline Canadian customs requirements with your pax...especially that no handguns can be brought into Canada without very specific procedures being followed.
U.S. Customs and Border Protection - Contacts