The lesson
Wrap the bid in the buyer's own shipping material: a kraft-paper cover makes a proposal look like a parcel before anyone opens it.
Kraft-paper cover with a black band down the left edge repeating "FUTURE. FORGED. FOR ALL." in white with a yellow "Pittsburgh, Pennsylvania" line. Right of center reads "Proposal for", then "AMAZON HQ2 • PITTSBURGH" in heavy black type and "October 2017", with "CONFIDENTIAL" at the foot.
Don't borrow the Amazon-parcel look unless your buyer has a physical identity to echo. Keep the idea of taking a material cue from the buyer's world.