Get legal advice!

Apart from this, it really is not as easy as one would wish. Personally I think your idea about a precentage of gross sales is not too bad, especially as you say they'd be happy with that. This guarantees that you'll get something regularly as long as they run/sell they product. The down-side is, if they stop selling it, or it sells badly, then you'll feel that.

I don't like the idea of getting paid in stocks. You can't go to a shop and get something with stocks. If you want to sell them to get to money, you are dependent to a certain degree on their market value - which will tumble at exactly the wrong moment.
On the other hand, stocks may - note: may! - be an investment for the future, depending on the amount you get, and the health of the company. Oh, they're in IT? Forget what I just said...

Selling your code for a one-off payment may seem attractive for the moment, but once the money is spent, it's gone. And I feel that a large sum at once can sometimes get to your (my...) head. Having said this, if you can live off your job well enough then this might not be an issue at all. At the same time, you get something regardless of whether they can sell the product or not.

All these options may apply for you or not. Whichever you choose, make sure you get legal help to make steadfast conditions and write them down in a contract!

The other thing is, how much do you want to charge? This question is always hard to determine. I usually tend to underestimate the value of my scripts. I think the way I would do this is, see how much I'd charge, try to find out how much they'd bee wiling to pay (which ideally would be more than what you'd charge, but don't ell them this (:), and then find a sum which is somewhere in between.

Hope this helps. All the best!

--cs

There are nights when the wolves are silent and only the moon howls. - George Carlin