I'm not sure if you considered this and your payment model made that easy or even possible, but I would guess that many people would like to try it first without paying anything and after that, they would pay whatever they think it is worth for them.
I would have done this. And if there wouldn't be an easy way later on to pay/donate, I would have just leave it unpaid.
I've been toying with the idea of making a simple service like that. You can download file X for free, only by leaving your email address. Then, after a few days/weeks/months, depending on what the author wanted, the service emails you and asks you to pay for the thing you downloaded, and gives you a link to easily pay.
I've found that I'll definitely pay for things after I've tried them, but I'm more hesitant to do it before.
Yeah, but that's bordering on "scam". The service I'm envisioning is more "reminds you to pay after you've used something, if you found it useful", and even the e-mail address could be useful to the creator of the thing (for further marketing).
You could even also opt to not share your e-mail address with the developer by paying directly, before accessing the thing at all.
I would have done this. And if there wouldn't be an easy way later on to pay/donate, I would have just leave it unpaid.