I hope your trip is for pleasure and not business. I looked into any possible options for this and was unable to find a way to get this done exactly how you are asking. We do not have the ability to future date a payment for one time only. However, there are other options:
¤ You can pay before you leave (or any time after your bill prints)
¤ You can pay on the day you would like by dialing *3 on your phone on that day
¤ You can go through sprint.com to pay on the day of your choosing
I know these options are not exactly what you were looking for, however they are the best I can do. The last option would be to sign up for auto payments, though this is a process that can take a few days before it goes into effect. I am more than happy to provide the steps to sign up for this if you would like, just let me know!