How to let customers set their own payment amount?

[WriteOn]

We are selling a rather expensive product -- actually an investment vehicle of a particular sort -- and we want to allow people to enter the amount that we will auto-debit from their bank account each month, or week, or at the interval they see fit, in a kind of layaway plan towards the purchase. I have set up the products so that there is a weekly product, a twice-a-month product and a monthly product. The fee we will earn from offering the plans is the amount listed as the price for each product.

 

Now here's the issue: I would like to set up, probably as a product attribute, the place where the customer chooses how much they will have us deduct from their account at each scheduled interval for the layaway towards the actual purchase of the goods. However, I would like the customers to be able to enter whatever precise amount they choose, rather than us setting up an ungainly large drop-down list of options.

 

Is there a way to do this? Thanks in advance.