How do I set a "donation" condition or variable-price-with-minimum on a product?

Just installed Zencart the other day. Not having a problem explicitly, but I had a question about pricing and I couldn't find it anywhere else:

I have the ability to make products free, or set prices. I would like to allow customers to pay any amount they would like for a product, possibly with a preset minimum, possibly with no minimum but a recommended price.

I'm doing this as part of an attempt to see what fans will pay for a virtual product. Basically is there a way to make the price customizeable by the customer at time of purchase, thus integrating both donations and product delivery? If not I'll just put donation buttons on the pages for each product, but it'd be neat if people could input or I could set a minimum and let people pay what they'd like.

Thanks in advance, and sorry if the answer is obvious.

There may be a 'make an offer' mod that you could use or adapt, check the Free Add Ons section