adding a payment system to a cart-less site. how easy?

[unwashed potato!]

i'm considering adding something like worldpay online payment gateway so that i can get customers to pay a deposit and even the invoice in full if they wish.

i wondered how easy if at all possible this would be for a static completely custom made site that doesn't use any cart system? i do have an online quote system, so ideally would like to tie it up with that