I believe if you send it in Dollars, they get the Dollars amount converted automatically to their currency, so don't worry about making a big donation, it wont affect you, it only occurs on the end run of the process.
If you donate $1, and the selected currency is '$', the OFPEC fund, unless changed from the days I used to do it, would get roughly Â£0.50p, after conversion, which is equivalent to slightly more than a bar of chocolate in British terms. I don't know if HuBBa has changed it to his strange alien money that they use over there yet, I presume he would have.
Best piece of advice I can give, is put a '1' then lots of '0''s.
N.B - Quick aside, remember, if you state in any form of transaction, that you wish 'x' amount of funds to be debited to another source, it doesn't matter what
that company intends to do with it, but they can never
remove more from your account than you stated on that form, including the currency format.