I substituted milkshake for xxxxxx
The server asked if I wanted to add a drink to my order but he did not mention anything specifically. (Needs comma between order & but.)
He thanked me but did not invite me to return. (Needs a comma between me and but. He is the subject inferred in the part after "but" so it is a compound sentence that needs to be separated by a comma.)
He smiled frequently and was personable.
I waited 15 seconds to order and was not initially offered a sample or a suggestion of what to order. After I received the food, the cashier asked if I wanted to order a drink and offered me a sample of milkshake. (These 2 sentences should probably be broken up into 4 or commas added for the same reason as above.)
The milkshake's were attractively displayed in a cup. (A plural of a noun does not need an apostrophe. It is not a contraction nor does the milkshake possess anything)
This is a mall location with no restroom. There was no banner on the front of the counter.
The milkshake's were not priced. (Again, no apostrophe.)
There were no prices shown for the drinks. They were nice and light and a delicious snack without being too filling.
As others have said, though, if you got paid don't sweat it!
Kim