not sure if it is possible, but what about an email to invoice gateway.

have an email address invoices@xero.com
i can email any invoices i get to that address and it automatically populates a draft invoice in xero, based on the from address of the sender

obviously you would need to have a bit of security, like a cryptic email address to send the invoices too, and it would be difficult to parse all invoices formats, but not impossible.

even if it pulled out the generic details of each invoice, eg invoice no, from, date, total, it would save a little bit of typing