On the Travel Service plane, we use the Soft Drink...

On the Travel Service plane, we use the Soft Drink + Spirit offer at the price of 4.20 .
We order 4x, calculating that we will pay 16.80 .
The bill is 17.20, and the steward says: because it beat me out
Apparently only 0.40 but that's my 40 cents!
Seemingly small, but still a scam.
Or service incompetence.
And the service is sluggish and rude