Woot offers 3M Indoor Window Insulation, Insulator Kit for $7.99 (reg. $37.13) free shipping with Prime. 

Product Description:
Available in a variety of indoor and outdoor window sizes. 3M Window Insulator Kits keep cold drafts out and warm air in through heat shrink window film and Scotch Window Film Mounting Tape. 3M window kits have the clearest film and stay up all season long with Scotch tape technology. Scotch double sided tape holds firmly between films and surfaces.

• Saves energy and reduces heating costs
• Reduces condensation and prevents frost buildup
• Quick and easy installation
• Fits one XL 6'8" x 19.5' indoor window