If it's anything like I had to do with a old ATT account, they will pro-rate your current months bill (as in how much days you are into it) then add in the ETF on top of that. I was able to pay for mine in-store at ATT when I cancelled the contract. Verizon shouldn't be any different I suppose. Just get the receipt that shows you payed them for your records in-case something goes south.
Yep, that's exactly what the Verizon lady told me when I finally found the right number to call. She wouldn't let me pay the ETF over the phone and said it will be on the next bill since I already paid for this cycle.