Don't commit a crime is where I would start. When they are open, go in and let them know how much you picked and pay then.
I would say the best way to do this would be to pick early in the morning when it's cool, and then as soon as the farm stand opens you can go and have your picked berries weighed and priced accordingly.
Just stop back when they're open and give them whatever you think appropriate. They'll appreciate it!
Thanks! Your answer is awaiting moderation.