Customer Service 101: Greet your customers. Preferably when they walk in the door. Don't toss receipts and cards on the counter, especially when their hands are extended to receive said items. Don't shove the bagel across the counter towards them, especially if their hands are reaching out. Thank your customers for visiting. Don't dismissively walk away without a word.
Change this and you'll get a return customer. Not someone who tells friends and neighbors that "the bagels are okay, but they've horrible customer service."