Of course you were being unreasonable @crisikrinkles It's a post office, you KNOW they would have had 'foreign' currency.
Something similar happened last week in the post office in my market town. 15-16 people (me in the middle somewhere,) and 3 windows of 4 open. They have 3 in a row, and one at the back on its own.
This woman trots in with a letter, and waltzes to the back of the post office to checkout 4, and says 'oh for fuck's sake!' She was disgruntled because it was closed, and only checkouts 1, 2, and 3 were open. Did she think checkout 4 was open and we were all so stupid that we didn't notice?
She started flapping her arms at the women serving at the other checkouts, and one of them said 'yes?!' The woman said 'I only want one second class stamp, could you just give me one, I have the right change here (87p.)' A woman behind me said 'I only want one stamp too, but I'm not trying to push in front of 15 people!' 
The checkout lady said 'sorry you will have to wait' as half the people in the queue scowled at this woman. 😆
The woman said 'fucking shithole!'' and stormed off. Ridiculous woman!
I don't know why people like her (and the OP and anyone supporting her) think their time is more important than anyone else's, or why they have some God given right to push in, and butt in when someone is being served with their pointless 'questions!'