I too "financed" (i.e. spread the payments) on my new iPhone when I upgraded last month. I was planning on paying it off at my local Apple Store but my local store told me that I have to go through the bank that Apple used to finance the phone. So that's a no go.

I just called a local AT&T store and told them about my situation and they said I can pay it off at their store. I specifically made sure they took Apple Pay. So I'll be heading there. After I pay off the phone, I will inquire about prepaying my future cellphone bills and report back.