Envelopes which are about to receive developed photographic films and/or prints are advanced toward a stacking station in a predetermined orientation and each envelope is provided with one or more fold lines extending in the direction of advancement of envelopes. The fold lines serve to straighten the envelopes which are delivered in rolled, creased and/or otherwise deformed condition such as could interfere with predictable stacking of envelopes in a predetermined sequence which is necessary to ensure that each envelope receive a proper set of film frames and/or prints for shipment or delivery to the parties whose names and/or addresses appear on the envelopes. The fold lines straighten the originally deformed envelopes (which are in random distribution with non-deformed envelopes) for an interval of time which suffices to ensure proper stacking, even if the envelopes exhibit a pronounced tendency to reassume their deformed state. A properly stacked straightened (originally deformed) envelope is normally prevented from reassuming its deformed state by the weight of envelopes which are deposited above it.