Try reviewing the settings of Quicken to use IE as default browser and OE as default mail client. You should be able to send the invoice within Quicken then your OE should automatically launch the New mail windows (if set as default mail client) with the invoice.

If the above failed or not possible, how about right-clicking on the copy of the invoice (from Window Explorer) then select Send To>Mail Recipient?

BTW as per Google Group search, Quicken will use MAPI (a Microsoft