I've been using K-9 Mail for years, I love it, great work! If you can spare the time, can you please add a simpler way to donate? I really don't want to create yet another account (liberapay?), please just let me pay with paypal directly.
But I can really recommend creating a Liberapay acccount (it accepts PayPal) as many FOSS projects use it to receive donations and Liberapay is open source itself and run by a non-profit. Let's get K-9 to its goal of $1000: https://liberapay.com/k9mail