I routinely travel 600 miles each direction to visit family about three or four times a year. I also make a habit of shopping on my way to a destination so I'll be able to go back to the store on my way home.
I started paying for my fabrics, then walking to an empty area, pulling the items out of the bag and checking them against the receipt before I walk out the door.
The reason I formed this habit was that when I was travelling and stopping at quilt shops I twice realized that I'd been charged $15 or $20 extra. It seems that some shops have a habit of overcharging people who are shopping there while travelling. They seem to think that it's too far to travel back there to get a refund.