Note that they "reserve the right to cancel" your holiday. They may not do so and possibly won't immediately anyway. But if they did, it is stated in their Ts&Cs and you have nothing to fall back on. I agree with Luke, I would phone. If you put it on a credit card and then pay the card a week later when you get your cash you should incur no interest charges, depending on where you are in your billing cycle.
Another point to consider is that underpayment and non payment are very different and have different consequences. An agreement to make an interim payment and an undertaking to produce the balance by an agreed date and then ensuring that you duly deliver as promised, is most times perfectly acceptable. But you need to agree this with the company.