If you have PayPal Standard as an option for a Shop store, any confirmation email you have created for the form will be sent when the visitor clicks the Go To PayPal button rather than after payment is completed directly with PayPal. If they abandon the transaction with PayPal, they will have received the confirmation email even though they haven't completed payment.
If you wish to only send an confirmation email after the payment has been completed directly with PayPal and you have Salesforce integration enabled, we suggest setting Send Email to No for the Shop store and creating a workflow within Salesforce that sends the email when a Lead record for the transaction is created.