Normally you depart your own country from a customs airfield or one “notified” or on concession. Your local AIAP entry will/should tell you for your specific airfield.
You fly to a customs airfield in the destination country (or agin, one that is concessioned) - the whole process is covered by a General Declaration submitted to the authorities in advance.
Whether you are being paid or not to fly the aircraft has no bearing on where the aircraft can go once it has been through customs at the other end. In simplistic terms if it’s the bosses aircraft and no person on board is being charged for travel then it is not considered a commercial activity.
if it’s the bosses aircraft, it could also be subject to less regulation than it would if operated by an AOC holder and more destinations “Open” to you.
Unless your boss is travelling very regularly then chartered aircraft will be cheaper to operate/own.