Customs people are human and if they recognize your intention is a donation, they usually turn their eyes. BUT you might run across one that got up on the wrong side of the bed and wants everyone else to have a bad day too. I once heard a customs person say to another......lets search everyone with a mustache today ...no reason, just because.
Years ago I brought a whole duffel bag full of deflated soccer balls and a couple air pumps for Preachermon(if any one recognizes), he helped with a youth after school program. Since it was a pretty sizable extra bag, I went to the declare line. The customs guy looks at me and says...you selling these? No sir, its for the Negril Police Boys program.....waved me through.
Sometimes there's no rhyme or reason.