Self-Sealing Security Envelopes – 500 Qty. Quantity: 500 envelopes per box. Why Choose Our Self-Sealing Security Envelopes?. Keep your correspondence professional in sleek, high-quality envelopes. Top Window: 7/8″ x 3-1/2″ (Positioned 5/8″ from the left and 2-1/4″ from the bottom).