A method and a configuration provide dynamic control of a liquid supply for a moisturizing storage device for sealing glued edges of a envelope flap of letter envelopes. Once a measured value has been measured for a sealing liquid which is stored in the tank of a moisturizing apparatus, the type of sealing liquid that is used is qualitatively analyzed on the basis of the measured value and of at least one material parameter as a comparison value. The amount of liquid stored in the moisturizing storage device is then measured by at least one further measurement to allow dynamic control of the liquid supply to the moisturizing storage device as a function of the material parameter and of at least one measured value, which is related to the liquid consumption, in the result of the at least one measurement of the amount of liquid stored in the moisturizing storage device.