If you are under some hardship, and you simply cannot afford it, (death in the family, loss of job, dissolution of marriage) write them a letter explaining the hardship and maybe- just maybe- some kind soul will take pity on you and terminate the sale. But if it's just a matter that you don't WANT to fulfill the obligation, I'm afraid you have missed the opportunity to undo the purchase and if you choose not to pay for it, they are within their rights to collect what you owe.
Good Luck!
Jim