"Bundling" plugin for buy-one get something free?

My client sells yarn. And knitting patterns. They'd like to combine the two ("bundle") so that customers get a free pattern if the buy the requisite number of skeins of yarn to create that pattern.

So -- are there any plugins out there that would do this? Or, is there a way to require a minimum purchase of product to get something free?

I have considered creating an entirely new product -- the "bundle" that has the requisite amount of product and a free download, but I'm wondering if there's a way to let the customer put this together on their own.

I think this does something like that:

http://www.zen-cart.com/downloads.php?do=file&id=201

I haven't used it, and don't know if it works with 1.5.0.

Thanks -- I'll check that out.

John