LUX 80lb 4 1/8 x 9 1/2 Square Flap #10 Envelopes Quantity = 900. Text Paper. LUX® Square flap envelope in natural color features a peel and a press flap for secured sealing. #10 (4 1/8″ x 9 1/2″) Envelope containing 100 percent recycled content is laser/inkjet compatible, sold as 500 per box.