There are a few ideas on where the old adage, "mind your P's and Q's," comes from but the most commonly accepted stories have to do with old printing presses and beer. There was a time when each letter had to be manually placed in a printing press and they had to be in reverse. Imagine the potential confusion when setting lowercase p's and q's! So "mind your P's and Q's" was a reminder to the typesetters to take care when setting these letters.
Also, bartenders in English pubs of the seventeenth century
would keep a watch on how many pints and quarts their patrons drank so they would know how much to charge. "Mind your P's and Q's" became a necessary reminder to keep track of how much one has consumed!
No matter which origin you prefer, the saying has come to mean that you should be mindful of the details; like your customer's travel details and your agency tasks. [more]