In the 1920s saving a few centimes had meaning, as this advertising postcard which I recently acquired nicely illustrates.
Posted on January 9, 1925, from Molsheim, a town in the Alsace, the card was sent as printed matter (Imprimé) franked with a French 10-centime Pasteur definitive [Scott No. 185] and addressed to Ettelbrück in Luxembourg's Ardenne. Upon arrival on January 10, 1925, it was taxed 30 centimes for being underfranked (probably the printed matter postcard rate had recently increased to 15 centimes), and a 30-cent postage stamp was affixed and cancelled prior to delivery [Prifix No 14]. The amount of the deficiency is not indicated, but at the time UPU regulations provided that such a deficiency was to be doubled, with a minimum tax of 6 cents. When delivery was attempted, the recipient (perhaps noticing that it was just an advertisement) refused to pay the tax, as the postman has noted in manuscript at the upper left. Since the tax could not be collected, it was cancelled by application of the red oval DÉBOURSÉ marking (shown below), after which the card was returned to the sender as shown by the blue crayon notation "retour."
Upon its return to Molsheim on January 12, 1925, the French postal service reimposed the 30-centime tax as evidenced by the 30-centime French postage due adhesive [Scott No. J34], which it presumably collected from the sender when the card was returned. The card (shown below) has a visual appeal that complements the postal history it exudes. The red DÉBOURSÉ mark and the red French postage due stamp are seen at the top center "framed" by the green Luxembourg postage due stamp on the left and the green French definitive on the right..
More examples of the use of the DÉBOURSÉ marking are shown in an article I wrote for Castellum a couple years ago: "Use of the Oval Déboursé Marking on Mail with Postage Due that the Addressee Refused or that was Otherwise Undeliverable," Vol. 7, No. 1 (June 2003), pp. 2-6. Castellum is the quarterly journal of the Luxembourg Collectors Club (http://www.luxcentral.com/stamps/LCC/).