Thomas, if you're looking for a way to generate, attach and email documents automatically without writing any custom code, then have a look at these two ISV products:
MSCRM-addons.com AutoMerge
http://mscrm-addons.com/Products/AddonsforMSCRM2011/AutoMerge/tabid/169/Default.aspx
XperiDo Server-side Document Creation Add-on for Microsoft Dynamics CRM
http://www.xperido.com/xperido-server-side-document-creation-add-microsoft-dynamics-crm
It is possible to build the invoice sending process without add-ons, but you'll need to develop a plugin that connects to the report server that creates the document. The reason why this is not simple nor an out-of-the-box feature is simply that Microsoft Dynamics CRM and SQL Server Reporting Services are two completely separate products. Yes, CRM uses SSRS for reports, but in practice it's a bit too heavy a solution for meeting this kind of everyday requirements for sales document creation in most cases. Word Mail Merge on the other hand is a client-side solution, which can't really provide a sufficient level of automation for streamlining the process.