QuickBooks Online allows you to send transactions to your customers so they can see what they owe.
Once you click the Send and close in the Send email page, there isn't an option to cancel it. On the other hand, you can hit Cancel if you're still on the same page. See the attached screenshot below for your visual reference.
You'll be routed back to the transaction page where you can select Save or Save and close. It will just record the entry in QuickBooks.