I’ve done a 3D design, can I include the 3D preview in the customer proposal document?

Yes, select Customer Proposal from the Tasks drop-down menu. In the bottom section titled Overview images you can select the 3D viewpoint you would like to show on the proposal. 

Note if you change the 3D design or panel layout in your project after generating the 3D image you’ll need to come back to this task and regenerate the image to be used in your proposal.