Alternatively, perhaps you could set up mailboxes for each user that can be entered as the billing address on the individual services.

eg: On my Freshbooks/Chartbeat/Less Accounting/whatever dashboard, I put in "gareth+somerandomkey@saasly.com" as the billing address, then all mails from that service go directly to you - you may even be able to attempt to parse arbitrary services, looking for a "Total: $n.nn" type string in the e-mail, or set up per-service parsing.