A1 White 3 5/8" x 5 1/8" Square Flap, Invitation Envelopes. - 100 Envelopes (White) Inkjet & Laser Compatible. suited for RSVP cards, invitations, announcements, and notes. 100 envelopes per pack. A1 Invitation Envelope (3 5/8 x 5 1/8) in 24lb.