One thing that I have learned to do with shop situations like that is to listen to music in my earphones if I’m ever out and about getting things. Keep them in the whole time until u are about to be served then pause music and just speak when you are being served. Works for me anyway because when I was younger I dine exactly what you did. Played out what I was gonna say over and over and of course I stuttered when it came to it. Hope this helps