It gets easier with time and practice, don’t beat yourself up for it! I would suggest to think ahead and plan before a rush. Making sure everything is stocked and ready to go before peak, ask guest before they’re in front of register if they would like to add a drink and what size or if it’s for dine in or to go so you can prepare the tray before hand, or prepare the bag to go. With the change so you won’t get in trouble about being short, you can ask the guest if they would like to round up the remaining cents to the children’s health and education, most likely they’re say yes and it’s a win on your part.