Quality Park #9 Double Window EcoEnvelopes, Mail invoices for your business and have customers send back payment in the same envelope, Box of 100 (24530) #9 invoice double window envelope (3-7/8 x 8-7/8 Inches) Double window placement designed to accommodate forms produced by most software mail systems. 30% post-consumer content/waste, EarthFirst recyclable window film Mail invoices for your business and have customers send back payment in the same envelope