Bringing back an item for a friend from the US. Nexus card or passport?

I'm going to my package place this week, and bringing back an additional item for a friend. My understanding is that I'm not allowed to do this on my Nexus card. Does the system care if I cross into the US on Nexus and back on my passport. I'm assuming the answer is no, but it wouldn't be the first time that an assumption bit me in the ass.