A method of merging postal articles with a stack of mailpieces that have already been machine-sorted into a delivery round comprises the following steps: inserting the postal articles in sequence into a sorting frame having slots; placing the stack of mailpieces on a merge table; displaying on a display screen of a monitoring and control unit an indication representative of a number of current postal articles to be merged, and of an insertion position at which the postal articles should be inserted into the stack of mailpieces; then manually taking from the frame the number of postal articles indicated on the screen, and inserting them into the stack to perform a current merger; and having the monitoring and control unit measure a distance before and after the current merger to detect any error in taking the postal articles from the frame.