One of the recipient sites (the one in Brazil) at
http://shop.wikileaks.org/donate#dpost says that you can put any name on "to" line of the envelope. (Maybe write something like "Free Speech Fund" or better, "Orphan Fund," on the "to" line but with the correct address.) (No, wait! Don't say "fund." Use something innocuous--a made-up name. Or you might just go ahead and write or type "Wikileaks," since the Feds no doubt have the recipient addresses.)
If you can't get a money order anonymously (say by paying cash), I'd say take the risk of sending cash. There needn't be any information about you on the envelope. Just be sure you address it correctly and get proper postage. And if you want to go that far, do the whole thing with gloves on and use back-slanting, disguised handwriting or an anonymous printer to print the address on the envelope. Don't leave a fingerprint on the money, the envelope or the stamp, and don't use your tongue to lick the stamp. And mail if far from your home. Better yet, put it inside another envelope to a friend far away and ask him or her to drop it in a mailbox for you.
That's the only way to remain anonymous to government/corporate snoopers that I can see, in this situation. They may be monitoring mail to these donation addresses (since the addresses are on line) but, even if they interdict your contribution, they won't have much of a clue who you are. Even if they put together your post here at DU with the interdicted mail item in your region, they still wouldn't have a direct connection between you and your donation. They would have to track your internet connection to your home--if you are using a computer at home--and how would they know that you put the donation in the mail? And if you have it mailed for you remotely by a friend in another state, there is almost no way they can connect your donation with your DU post. I suppose if they were truly after you and have tracked your entire network of friends and contacts, they could possibly put it together, but they would have to be highly motivated to do that.
I don't think they are going to go after small Wikileaks donors or make an effort to find out who they are (unless the donor makes it easy for them--then they might, just to keep their spying programmers busy). Maybe big donors will get that kind of treatment (who would be smart to make donations in small segments). There is huge traffic in donations and political opinions that the Corpses and the Government don't like. They would have to have a good reason to go after you amidst the Mount Everest of useless data they accumulate in their spying programs. If you are engaged in other activity that the Corpses/Government don't like--are an "Occupier" or are challenging Monsanto with an organic farm, or whatever, or have an activist history--then be more cautious.
I don't think you are violating any laws by sending a donation. But I am not a lawyer. Do not rely on me for legal advice--nor stealth advice. I am no expert in either field. But I don't think you are talking about that kind of harassment--a midnight knock on the door, arrest--but rather getting on a "list" for further surveillance and/or anonymous kinds of harassment, just picking on you and making your life difficult because of your political opinions.
I don't think you are being paranoid, by the way. I take your fear very seriously and you should, too. There are ways to harass someone that the victim might not even realize is harassment--loss of a job or promotion, loss of good credit, identify theft, a loan getting turned down, etc. It is smart to be careful and to realize just how vulnerable we all are to electronic and other malicious actions--merely for expressing our political opinions.
We should be asking: What happened to our democracy? My first answer to that is: Diebold/ES&S!