Whatever amount of product you end up buying, you gotta do everything you can to get it exchanged for moolah. Sounds like you'll be busy in the back and possibly have an inexperienced person or two on the front line.
If you have "new" help, you might consider tutoring your helpers (at least your point of purchase counter help) on being good salespeople. Make sure your help gets a taste test of all of the offerings so they can enthusiastically suggest items as add-on sales to the initial order. Suggestive selling. Have them steer customers to high profit items. Make it a game between counterhelp that they compete against each other by counting the number of orders they fill that only contains one item. Have an hourly celebration for the winner (multiple item sales) and start the contest over again to bring out the competition in a person.
I don't know if sweet tea is popular in your market, but it would have a good profit margin...every customer needs to leave your stand with a tea or lemonade in hand. "How 'bout a nice cold sweet tea to go along with that tasty sandwich".
Not trying to add to your inventory, but do you have a way to sell chilled or frozen candy bars? Maybe Snickers or Milky Way mini-bars? Everyone loves dessert! You could probably move mini-bars at 50 cents...might run the numbers to see if it is worthwhile. No one likes waiting in a second food line for a sweet treat...tie that thought into a final "super size" sales effort.
Sounds like an exciting, busy weekend surrounded by friends and loved ones...post up how it went!
UDS x 2...now they're reproducing!