Some different perspectives.....well, I don't know who the organization is but it could be that they are just slammed with work right now and it just isn't practical for them to stop and email every person that sends in a donation. Charity groups usually aren't exactly overstaffed, you know?
Also you commented that you were upset that you took the time to get to the post office and get them there. Is that not part of what you committed to do back in October? You were slow in meeting your commitment so maybe cut them some slack that they may have been running slow in responses?
Also consider if you truly just wanted to know if they arrived or if you really wanted some gratitude. Of if you wanted a formal thank you just as a matter of etiquette. There is no "wrong" answer.
Not saying I agree with any of the above necessarily but you asked from some different perspectives. I would recommend just trying to accept that they are probably over worked and if they have a lot of donations coming in right now I don't it should be a mark against them that they aren't sending out emails to every person who donates when something is received. And remember that no matter what, you are helping someone in need, gifting someone in need. Minor foibles in the organization don't change your good acts.