If you use TouchNet as your payment processor, any confirmation email you have created for a Soapbox Donation form will be sent when the visitor clicks the Make Payment button rather than after payment is completed directly with Touchnet. If they abandon the transaction with TouchNet, 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 TouchNet and you have Salesforce integration enabled, we suggest setting Send Email to No for the Soapbox Donation form and creating a workflow within Salesforce that sends the email when a Lead record for the transaction is created.