You can't. With BRP they require a one night deposit and the balance due at check in. Payment is due at check in, but they won't take your money any earlier. There have been several posts on this. A few people have found a CM who would take a credit card payment, but then they reported that a few days later the amount was credited back to their credit card.
I am doing a split stay at POFQ and CSR for Dec using the best rate plan, so got to pay two deposits, but they won't let me pay the rest. I also got a BRP for Jan and again I called (just in case the policy might have changed), but was again told the balance could only be paid at check in (in case the rate went down later). What I've read a few people are doing is buying Disney dollars, putting them up, then using them to pay for their rooms. This is what I plan to do. I like to have my trip paid for before I leave home.
Does anybody know how to make a payment on a ressie booked with BRP? We are booked in Jan. and could not make a payment on line. Called a CM who said we could not make a payment over the phone for a ressie booked on line, but we could not figure out how to do it and she did not know how -Help it's driving us nuts 
Thanks!
