Culio, you're at risk of getting "don't do it" warnings in big red text and so forth, when you present this question here. It's certainly not a new subject, as you seem to have noticed.
As Whopity correctly asks, why is it so much cheaper for you if you pay up front? It does not cost less to operate the plane because someone has paid up front, so what saving is being passes along to you? Or, is the pay up front price actually the fair price, and the pay as you go is inflated? Either way, it does not ring true for me...
A small discount for grouped payments, which simply reflects the savings in administration, and payment processing, perhaps, but this should be a small difference, not a big saving for you. What else in life are you invited to pay large amounts in advance for? Not much I can think of...
I run my business, with the expectation that clients will pay for services at the time they are consumed, not much later, and not much before. I suggest that a good business model for both sides of any service would be similar. If you need the savings seeming to be offered with advance payments, you probably should not take the risk! I wouldn't!