I would like to include a product option for donation, along with my other fixed price offerings.
Is there a way to do this -- so that the buyer can type in the amount of their choosing?
Thanks!

You could code in a separate PayPal button into the page of your shop.

You could probably add it as an Optional Field that they can choose it if they want to. Then just put the difference of the price of the item they are ordering and it will add the donation to the price of the product.

If you need an actual donation setup, create it as a multiple set of donation products. Put a Basic donation price in and use the Optional Fields for different increments.

No you can't do the text input so they can put their own in unless you do Eric's suggestion which works well too. But doing it using the Product as a donation setup works too. Just need to make a shipping and tax field that can correspond so they don't get those fees too. I'm sure it's doable though.