These don't really sound like 'save the dates' though, more admin.
Plus, for UK taxpayers, if it makes you feel any better, we're only paying for security on the day- the actual ceremony and reception are being paid for by the Queen and Prince Charles. Although I believe Kate's parents will pay for the dress.
I've just scared myself by knowing this. Is it made better that I only read 'Hello' when I'm at the hairdresser?!
As Larrabee said, the State (i.e.: the taxpayer) pays the Queen and Prince Charles (though PC also earns an income through his Duchy products). Besides, they have a 'personal fortune' that could easily pay for security as well. Just as anyone else would have to, if they organised an event large enough to need security.
Not to mention the income from the Duchy of Cornwall itself. I'm not sure it would even be possible for them to pay for the security. It is the type of event where it has
to be done by the state, they are the only organisation with the numbers and the experience to cope, and I don't know that private citizens would be able to contribute to the cost, even if they wanted to.
With regards to the faxes it does seem to me more like dealing with the formalities of a state occasion, than with the wedding itself. I have also heard before what someone else mentioned - you aren't actually invited/offered anything to do with the monarchy until they know you will agree! I think I heard it in the context of titles - you are not offered a knighthood or other honour, you are asked if you would accept it, then when you confirm that you would, you are offered it. Same sort of thing here, I would imagine.