I believe this is due to Mac El Capitan release.  This started happening to me as well after the upgrade.  Here’s a workaround I have. You have to check the “Save in Drafts Folder” . Then it shows it in Mac Mail with the attachment, and you can send it. […]

Resolution for 'bug fixed issue: 'The sales receipt is not being attached to the email to customer.' available: Yes.

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