You could make your own, it would have as much authority as the LOA.
Small shop owners don't care if you've got a letter from the queen, they will not want you doing ANYTHING in their shop and will tell you to leave/chase you out if they even think you are up to something!
I tend to avoid any task that requires
photos inside a cornershop - you'll get rumbled PDQ and asked to leave.
'introduce yourself to the employee' - They don't care.And will view you with suspicion.
Ask lots of detailed questions like "Do you stock brand x vape? Have you got a vape recommendation?" - no-one asks the geezer down the cornershop that.
Tasks I love
Outdoor photography - take your time and no-one bothers you.
betting shop tasks - they are weird places where EVERYONE is on their phone every 30 seconds.
supermarket tasks - Literally no-one cares!