A method of preparing batches of mailpieces using an envelope-stuffing machine. The method includes printing documents with each page bearing a first marking for causing the related document to be inserted into an envelope, and printing separator sheets each bearing a second marking for causing it to be folded into a predetermined particular geometrical shape; folding the documents and inserting them into empty envelopes having a common format; folding the separator sheets; and ejecting closed envelopes and the separator sheets folded into the predetermined particular geometrical shape. On being ejected by the envelope-stuffing machine, the separator sheets are interposed automatically between the closed envelopes as a function of the batches to be prepared, and the predetermined particular geometrical shape resulting in the separator sheets projecting from the predetermined common format of the envelopes.