a printer apparatus has a contact pressure device for flat articles on a continuously moving transport belt , in particular letters in a franking and / or addressing machine . in order to increase the usable region and / or of the throughput of the printer apparatus , flat articles of different thicknesses are processed with predetermined speed , without slippage , in succession with arbitrarily small gaps between successive articles . an elastic , bellows - shaped , resiliently supported air bag has a low - friction , wear - resistant cover surface that is in non - positive contact with the transport belt . with its associated retention and air supply devices the bellows serves as a mobile contact pressure module .