In Shop → Products, create a product with a name, description, images and price (you can add a discounted price too), set its stock — globally or per size — and optionally give it a category.
Stock decreases automatically when a product is paid for online, or straight away for a cash order. Refunds can optionally restore stock. Items show as In stock, Low stock (under 10) or Out of stock.
Yes. On your booking page, clients choose Online to pay by card through Stripe, or Cash to pay at pickup.