If you have a GitHub account then you can use that to donate: https://github.com/sponsors/cketti

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