A recording media sheet processing system includes a folding device including a folding unit to fold a sheet and a squeezing unit to squeeze the folded sheet, an insertion device to insert in an envelope an enclosure, and a controller. The folding device. The controller includes an envelope selector, a selector for selecting whether to fold the sheet and a folding style of the sheet, a first storage unit for storing a folding-related equivalent quantity into which a quantity of each sheet is converted corresponding to the folding style and the number of times the sheet is squeezed, a second storage unit for storing a maximum quantity of sheets insertable, a calculator to calculate a total converted quantity of the enclosure, a determination unit to determine whether the selected envelope type accommodates the enclosure, and a squeezing setter to set the number of times of squeezing.