If you need to leave a sale and preserve the cart, simply click another tab or "Cancel" on the top left of the screen. The customer's cart will be saved until the cart is manually cleared or the transaction is completed.
When you start a new sale with this customer, you will be presented with a screen showing their previous cart and two options:
Continue the sale by clicking Reload, or start a new sale without the original items in the cart, select Ignore.
The cart is synced across the platform, so if a customer starts a purchase online and abandons it, this screen will display when you ring them up in the studio.