Google is your friend, my friend, and ‘Google it’ should be your first course of action. I found these instructions in 5 sec by googling: ‘PayPal micropayments.’
In short, contact them through Help and they will do this upon your request. There is no form or anything of the sort. A word of WARNING, though — doing that only makes sense if a vast majority of your incoming payments are below $15 since this fee will be applied to all of your payments and if they are higher than $15, you’ll end paying even more than you would with a regular account.
What’s more, once you have your account turned into a micropayment account, there is no way back. They will simply not revert it back. I had to create another account so that I now have two — one for micro and the other for higher transactions.
I don’t think PayPal is short on money and that your donation would help them improve their UX (it’s a mind rather than money problem), but I sure could use a donation because that helps me write more helpful posts!
You could, for instance, treat me to a coffee (and get my audiobook in return, yay!) or become my patron on Patreon (strating at $1 and get access to tons of patron only posts with helpful insights and all of my ebooks and audiobooks for free, which is an even bigger yay!).
I totally understand if that’s too much to ask, though, and if you’d like to enjoy free posts like this one that’s an option too — but in that case, may I suggest you subscribe to my newsletter, since Medium makes it so very easy to miss new posts? I do not spam and I usually send one newsletter with a selection of the latest stories and insights per months. So there :)!