Your mailing address should be on all your invoices and proposals. To add yours, go to Settings > Mailing Address.
While we call it "Mailing Address" you can actually add any text you'd like to here. Some add a line with their phone number or website address. If you're a nonprofit, maybe add your Tax ID. You can even add your company motto if you'd like! These will all show alongside your logo on invoices and proposals.