Can I add an attachment to an invoice?

Ah, no, 'add attachment' doesn't mean adding a separate attachment. It means adding your sales invoice as an attachment to that email :)

Otherwise, your customer will have to click on the link in the generated email to open the sales invoice. You should read the email when sending a sales invoice via Dexxter. If you click 'add attachment', it doesn't have to be via that link, but it will be attached to the email as a pDF. But I understand the confusion :)