The disclosure is directed to a semi-automatic mail insertion machine which assists an operator in placing materials into envelopes (26). The machine includes a horizontal conveyor (20, 21) which, under operator control, incrementally advances the envelopes (26) to be processed from one processing station to another in succession. An envelope stacker (25) is positioned above the conveyor (20, 21) and a feed means (55, 56) feeds the envelopes (26) to the conveyor (20, 21) prior to each advancement of the conveyor. The envelopes (26) are conveyed to a first process station (30) which automatically unfolds the pre-folded unsealed envelope flaps. The envelopes are then advanced to a second processing station (35) which opens the envelopes and presents the envelopes to the operator in a manner which permits ready placement of materials into the envelopes. A third processing station (45) may be provided which weighs the envelopes and the material therein to facilitate the sorting of the processed envelopes by weight.