buyer beware - 2 scoops is is not charge as double scoop @ swensen's - Caveat emptor

I must say we had a interesting event at Swensen's over the weekend.
We frequent Swensen's almost on a weekly basis. We usually ordered 2 ice-cream to share among 3 persons. (The evil wanna eat but still afraid of putting on weight)
Our typical order is always plain scoops of ice cream for the one who does not like any topping at all and an other type like cookie summit, coit tower, mighty nutty etc.
This time, we asked for 2 scoops, one vanilla & one macadamia flavour. The other order is a might nutty. The waitress took it that we wanted 2 separate single scoop ice cream and proceed to send us in separate cups. Unknowingly, my boy ate up the 2 cups of ice-cream  and I did not give much thoughts to it except commenting that 'so funny, why she must separate the ice-cream.

After settling the bill, I then realised that we were charged 2 single scoops ice cream of $4.50 each (total $9) instead of the double scoops which cost $6.50.
What I wanted to put across is the poor service then took place. I feedback that I wanted 2 scoops and not 2 single scoops, however because I have made payment, the cashier refused to reverse the bill. She asked if they were served in 2 single cups and i replied yes. So she insisted that I ordered wrongly and hence it was my fault and there was nothing much she can do. She even had the cheek to said I must order using the word DOUBLE and not 2 scoops.
What kind of service is? I am utterly speechless.

It gave me a feeling that double scoop order will be lesser than a single scoop!!!
Is that why they won't reverse the mistake?

I wondered if I have ordered a double and separate them, what would they charge me?

It is a buyer's beware market !!! Caveat emptor!