My ideas on trying to sort out the Sort.
Ideally you would like to have everyones baggie open all at once to do the exchange. This would need about an A3 size (approx 2 sheets of letter paper side by side) per person - an awful lot of table space, then I considered using chairs, 1 per person. Still huge, you would need a 615(plus) seater auditorium. However it might be more achievable in a church, using the pews.
With each person's HST's, postage box/bag & return label on each chair, remove the HST's from the baggie and leave them on the chair, taking the baggie ,then go and fill it with the correct number of HST's from every other chair but the original. When it is filled, return it to it's packaging/return label and turn it over to show that it has been filled. Then move onto the next baggie, and so on.
Whatever system you decide upon, write it up on a large sheet of paper and place it where everyone can find it. You could also make small directions sheet for everyone taking part to keep with them, so that everyone is working to the same system and not changing it part way through
Still you've lots of time to sort through all our suggestions
Rosemary
PS you could also designate one person to be the 'Official Panicker' - they do all the panicking for everybody else while everybody else gets on with the job in hand. That way you don't have more than one person panicking at a time