I'd like to encourage using of Flattr for supporting Calibre. I know it was mentioned before as alternative to PayPal, but it was not argumented at all, so please don't take this as plain insisting.
As I have relatively low income and many projects to appreciate, for me Flattr is not only affordable, but convenient as well. I'm sure, many users find it the same.
Another personal reason is that I use a separate credit card for online payments, which is mostly kept empty for security reasons. So it is technically time-consuming to bring any support for you - about as much, as to write this message - so for now please take it as my form of support
Its no problem for me, if I have no convenient way to donate - but Calibre project might find use in making it convenient.