Your company could implement a centralized application to store and manage these documents, basecamp, sharepoint, etc. If you just send them via email, the customer/vendor doesn't have a easy central location to search and retrieve information. What if someone leaves one of the companies? Then you have to resend the emails, or their IT dept needs to consolidate email accounts and access. This way, the new employee gets a login and automatically gets all the invoices and documents. Notifications can be automatically sent via, IM, text, snail mail, or even automated phone calls. It won't be easy to get rid of email, but its definitely possible.
