As for their idea, I came across an interview with the show creators,
Do you actually know what idea Donna wants to pursue, or did you leave that open for yourselves as well?
ROGERS: That’s a question that’s coming up a lot and may come up for time immemorial. I think if you’re looking closely, there are a couple little nods to what the idea might be, but I also think there are a couple different answers that one could arrive at given the pieces there. We didn’t want it to be the end of The Sopranos where it was like the guy in the Members Only jacket and like “did the cop kill him?” — like where there are lots of competing theories and one is right. I think when you’re ending a series, sometimes the sweet spot is to suggest some possibility that the audience can kind of fill in the rest of, and I think Donna and Cameron’s idea might fall in that category. Is Joe going to stay in that school forever? Is this the real Joe? Have we finally arrived, or is this just another iteration of that guy? I think that you bring that answer to that scene. In that same way, I think Donna and Cameron’s idea and the viability of it going forward is a thing I’m happy for the audience to fill in at this point.
I’ve heard someone say it might be wifi. The cash register shot might mean some sort of paypal, ebay, or amazon type thing.
When the show first came out they made some spotify playlists that fit each character. Worth checking out if anyone liked the vibe of the show.