Why not set a forward filter based on the subject on your end?
Been doing that for years...
Could do (and have in the past). But it is a workaround, and takes extra effort.
A couple of our suppliers do have the separate-email-for-invoices-only thing, and it really is super convenient to know that any changes they make to their invoice emails (subject lines etc) won't break our forwarding rules.
Also the forwarding rules don't survive email migrations (which I admit is a super infrequent thing to do). When we migrated from Google Apps to Office 365 we lost the rules we'd previously set up for this type of thing.
We use reciptbank (that has an inbound email address for forwarding invoicss to) for invoice processing into our accounting system.
Currently invoics go to our accounts@ email address (along with all other accounts related stuff) and the invoice specific emails are forwarded onto receiptbank manually.
Would love to be to tell the saas platform to split that out into two different email addresses.