As of the time I'm writing this, your donation page still says:
What is Philsophy Worth?
Number of Podcasts: 4
Donation Request: $2.00
All support is deeply appreciated, thank you so much for your help!
The most simple explanation as to why the donator gave $2.00 is he listened to a few podcasts, liked them enough to want to give you money, and picked the exact amount you asked for!
So revise your band example to include a sign in front of the tip jar that says "Suggested donation: 50 cents per song" and ask yourself if you'd feel the need to explain yourself after only having listened to a few songs.
And revise your store example to include the sign that's in almost every existing convenience store: "Minimum credit card purchase: $x". Convenience stores and other businesses that deal in small transactions do things to protect themselves from getting eaten up by processing fees. I don't hear or see you taking personal responsibility for allowing a transaction you say you don't want. You're a former CTO, aren't you? How much time would it have taken to modify your donation page/code to prevent donations under a certain amount? I can't believe it would take any more time than what you've already spent on this issue.
I also don't know what to think when I read your post here compared to your initial reply on facebook:
I know, I know, it really does sound ungrateful, I get that, but I did want to express some frustration, because of course PayPal takes almost 20% of the $2, and then I end up having to track and record and summarize and report and pay taxes on $1.60, which I'm pretty sure ends up costing me money. I really do appreciate everyone's feedback, and I get that the comment came across as a little 'precious,' but really, I am spending enormous amount of money on this documentary, and I've never talked about small donation amounts over the past 6 years, even though it happened more times than I care to remember...
I don't see any curiousity about your donator's situation or motivations there, only how much money ends up in your pocket. If processing a $2 transaction really does cost you money, then there's something wrong with the way you're handling them. There are digital goods being sold all over the internet (songs, apps, etc.) for even smaller amounts at a profit. Again, I don't see you taking responsiblity for this. You chose your business model. You chose PayPal as your processor. You chose how to do your accounting and reporting.
What I'm curious about more than anything in this situation is how the donator felt (and anyone else who donated similar amounts) when you left your initial facebook comment, along with the follow up posts.