Great deduction!

When I figured out the solution, I was in the middle of driving.
I, too, realized the men didn't actually pay 27 shillings, they paid 25 shillings.
25/3 = 8 1/3
Then each had 1 shilling returned, so they paid effectively 9 1/3 shillings. That is 28, plus the 2 the waiter stole, =30