I dont meen Postiennakko, I think that is too risky. I have one shop though, that has it as an option and it works fine but if it is more common products then people don´t care about our loss.
I do meen similar to advance bank transfer payment.
I´ve made a simple system. When the customer chooses Ennakkomaksu on my site they get the information how to make the payment.
First directly in the confirmation page and then later in their email and also in their zen cart account is a bill kind of page where the information is. Everything works automaticly.
Then they open their net bank and makes a transfer as in any other payment of bills normally.
This bank button is kind of over charged. The customers have log in to the bank anyway. a bank button is like a link and not much else. Except it fills in the transfer form an no copy pasting needed.
but copy paste and writing is not a big deal I think. All people in Finland do that every day.