Very Bare-Bones: Invoicing emails have everything they need to, but they lack a certain je ne sais quoi.
Merchants CC'd: Currently, merchant accounts get CC'd on invoice emails. This makes some degree of sense, practically, and is consistent with other Phabricator applications, but feels a bit weird. Dropping them from non-approval emails or sending them a separate "merchant" version of the email might make more sense.
Subject: They generate with a subject like [Phortune] [New Invoice] Order X: Phabricator Instance which is, again, technically OK but not really great.
From: They should come "From" the merchant as an address display label. The currently come "From" the default address.