Fry has frozen himself multiple times, but he never seems to incur any sort of bill for this cryogenic freezing process. The future definitely still has money, so why doesn't Fry ever have to pay for this process?
To the best of my knowledge this is never addressed on screen.
- Off-screen, either Al Gore or the Nibblonians took care of the costs.
Both groups are fully aware that the fate of the entire universe depends on Fry remaining frozen at the cryo-centre. Neither group seem to lack the funds to ensure that this takes place.
- Off-screen, Fry took care of the costs.
Fry travels into his own past on at least three occasions. There's no reason to assume that he couldn't pay for the tube in advance. And yes, I'm aware that this is a bootstrap paradox. Go to hell, logic!
- The cryo-centre didn't notice or didn't care.
Civilisation has collapsed on at least three occasions since Fry was frozen. It's certainly possible that the cryo-centre's HQ (and its billing centre) were in a building that didn't survive.
Typically you would pay upfront for cryogenic freezing. The fact that he falls into them on accident multiple times, leads me to believe that since he was already frozen the people running the place must have assumed he had already paid. (Billing never talks to the scientists; a typical lack of communication between departments)