Part Payments and discounts

Hi,
I have found a few posts to do with part payments.
Mostly I am interested in doing this as we import products from the states, they yake up to 6 weeks to get here but we would like to sell them up front. So ideally we would like our customers to pay 50% up front and then 50% upon the units arriving in the UK.
As far as I can tell there is no way of doing this currently?
What makes it more complicated is that as a bonus for pre-ordering we would like to give a 20% discount (instead of the normal 10%).

Now if Zen doesn't support this at all I am trying to think of another way around it. I am a bit of a newbie though so would like any other ideas from people as to how to do this.

The main ideas I have at the moment:
Sell the full product at it's discounted rate (20% off then 50% deposit) and then contact the customer at a later date and get them to input a code that will allow them to buy a second product, not ideal as it then sells 2 products rather than one.

This could work if I can work it out.. Set up a new product called pre-order, this would be a virtual/downloadable product that allows the customer to put 50% down, this then gives them a code that allows them to buy the actual product at 50% off when it comes to making final payment?
Would this be able to be a single product or would I need to set one up for each of the real-workd products?

Final idea, just sell the item at 50% and then contact the customer at the right time and have them pay either by cheque, balance transfer, or through paypal without using the site. (we don't currently have a merchant account to take credit cards without paypal) Does anyone know if using paypal this way attracts any fees?

Any other ideas would be gratefully appreciated.

The website is http://www.stainlesstools.co.uk if anyone would like to look and get an idea of what we are trying to do..

Thanks in advance...