Managing workflow with multiple USB sticks

[oneeyedphoto]

So I've been asked once again to take photos for an animal rescue group I volunteer for.

The Paws Walk is a fundraiser to help raise $$ to contribute to the costs of rehousing unwanted pets. In the past I have taken photos onto my SD cards, edited images in Lightroom and the prints get done by the rescue group during the following week, which they receive $10 from pet owners who purchase them.
This time around they want to purchase blank USB sticks that they want to sell to the public with their images on them on the day - almost straight after I've taken them!

Logistically, I thought I'd ask fellow members here how they might manage a large scale workflow like this? (there's around 500-600 people that attend the PAWS day held over a couple of hours) but not everyone will buy one of course.

There is no booking times and I'm concerned how long people may have to wait for this technical process to occur.

Any suggestions welcomed.