The SCM has a validation method incorporated that, if I remember correctly, tests if the amount paid is at least 90% of the price set for the article. And out-of-the-box there is no option for the user to change the price for an article if the SCM is used to generate the PayPal links. So I doubt that the SCM is the best solution for handling donations.
In addition, I’m not sure if tying donations and access to some stuff is a good idea: Donations should be voluntarily ... if the users are only donating to get access to an ebook and they can modify the amount, I’d expect more or less of them entering 0.01$ as the donation amount.
What I would recommend: Set up some donator membership levels (e.g. silver = 20$, gold = 50$, platin = 100$) and put these for sale through the SCM. Then, have the SCM change the member group accordingly after successful payment. And give your ebooks as a bonus for these member groups, perhaps giving silver members access to one, gold members access to a pack of three and platin members full access to all your ebooks through clever setting of access rights to some blog entries.
Can’t speak for the license question, so.
-Markus