We are currently migrating a lot of transactional emails from a different platform to HubSpot. One of those e-mails is a order confirmation. Currently, it is not possible to send an attachment with a transactional email unless you use the SMTP API. But by using the SMTP API, we can not use HubSpot templates and have to use coded templates instead.
Please make it possible to send attachments with the Single Send API (Transactional Email Addon)
Hubspot advertises this costly feature with "send purchase receipt"...not mentioning here that you can't send the real documents is at least shady. Now we have to program a costly workaround for that. That is indeed disappointing!