If you run into a situation where you need to hold a customer's transaction for a brief period of time before completing it, you'll want to use the 'Save Transaction' feature at checkout.
Once you have a transaction loaded in the checkout tab that you want to save, elect the Save Transaction button in the bottom left. You will then be prompted to enter a description for the saved transaction. If you've already linked a customer to the transaction, their information will automatically populate.
The transaction will then be added to the list of other saved transactions, which can be found just below the Complete Transaction button.