Fixes T13341. Currently, cart emails (invoices/receipts) are sent to members of the associated merchant account. This was just a simple way to keep an eye on things when this was first written.
The system works fine, and recent changes (almost certainly D20525) stopped these emails from working (presumably because of the slightly weird merchant permissions model).
This could be sorted out in more detail, but it looks like the path forward is to introduce a side channel for email anyway (via T8389), and that's a better way to implement this behavior since it means the normal recipients won't see a bunch of random staff/merchant email addresses on their receipts.