How can I add an installment/lay-a-way feature?

I know this info gotta be here, but I can't find it with my search terms. Anyway, I want to offer an option for my customers to pay 1/2 down now and the remainder once their product is complete.

So I'm thinking of something like this:

They choose length $100
Color $50
style $20
ect
ect
Pay by installment plan (checkbox)

Now this is the tricky part. Where do I tell zen cart to divide their order in half and only charge them 50% of their total when they check the "pay by installment plan"

There is another way I can do it but I'm in the process of reading zencartoptimization so I'm all about the flow of things and less confusion.

TIA

How are you gonna charge the rest though? Which payment method are you using?
Paypal and most card merchants wont let you do that (what if you charge the customers more that you should?)

Well I was going to send them a paypal invoice for the rest. I use paypal for everything and all my competitors are giving this option (most use paypal too).

I guess another option is to add the installment plan as a single product, add all the attributes and set everything as free. That way I will have their order info in zen and just send them the paypal invoice separately.

I'm looking for options here because several customers already inquired if I offer this or not so those were potential customers I lost. Any suggestions?

If you have the ability to manually process credit cards, you could use the Installment Plan contribution.

Thanks, I'll take a look at that!

Well it doesn't work with paypal so back to the drawing board