My only experience has been with Verizon's version of the monthly pay program. They basically would only take a set monthly payment.. but what you are able to do is payoff the remaining balance at some point. So even if this was your only option, you could just put the money aside in a savings account and pay it off when it got to the balance. I don't believe most plans would support the equivalent of a mortgage extra payment though. Not sure if this was carrier specific or apple specific though.
Just my 2 cents... 32 GB without an SD Card seems very small for storage as phones tend to go. I would recommend the 128 if you can swing it.
Good luck
