Oh dear, no good deed goes unpunished!
In defence of your niece, I agree that she may have been worried, and if tired may have sent a quick (aka 'terse' email) and capitals are for emphasising.
I wonder if she is feeling a bit awful, but doesn't know what to say? I know that on the odd (I hope rare) occasion that I have been less than polite, I have wondered about apologising, as it seems to drag something relatively minor out to be chewed over.
I have a mantra that has served me well, though you may not agree with it. I think that as a senior member of the family, it is my job to smooth things over, to make overtures and generally be a role model.
So I would ring her (or email / message if that seems better) and say something like.....what a shame that our wires got crossed. I had hoped to surprise you....you must have been worried about the money going astray, indeed I felt quite upset myself.....
If she has anything about her she will get the message and be contrite, if not actually apologetic, and may seize on the excuse to say that she was terse because she was worried.
If not, I would write it off to experience! My father was worried about one of my nieces when she went to work abroad, at age 20. He sent her enough money for a return plane ticket 'in case you need to come home quickly' (several hundred pounds). She never thanked him - I think she was a bit distracted & careless rather than actually ungrateful, but he remembered. When he got a windfall, all the other grandchildren got a nice sum (over £1k) but she didn't! He didn't publicise that, just gave them each the money on the QT - they would have simply assumed she got the same.