Redirected mail is an inadequately explored area of postal history, particularly insofar as forwarding charges are concerned. During the early years of the UPU, the calculation was simple. If the first dispatch had been fully prepaid and the charge for the further dispatch had not been paid prior to the redirecting, then the difference between the amount of postage already paid and that which would have been required if the article had been originally dispatched to the new destination was charged to the addressee.
This cover nicely illustrates that calculation. It was originally sent on August 20, 1907, as a fully paid (10c) domestic letter from Anvers (Gare Central) to Louvain, Belgium, addressed to Monsieur Wilhem Willems. It was then redirected to the addressee at the Hotel Vincent Theato (shown in the photo below) in bucolic Grundhof, Luxembourg. If the letter had initially been sent from Belgium to a destination more than 30 km. within Luxembourg (in this instance, Grundhof), 20 centimes postage would have been required. Accordingly, the charge for redirection (10 centimes) was the difference between what had been originally paid (10 centimes) and the 20-centime letter rate to Luxembourg.
Although a parcel post agency had already been opened at Grundhof in 1901, the 10 centimes postage due for redirection was collected by the full-service Beaufort post office nearby, as the 10-centime postage due stamp canceled at Beaufort August 22nd documents. (Luxembourg had issued its first postage due stamps just 9 weeks earlier. )
The Beaufort double circle cancel also appears on the cover as a backstamp (shown below).
The second example is a commercial cover posted July 5, 1938, from from Zürich, Switzerland, to Linthal, Switzerland, at the 20c Swiss domestic letter rate, where it arrives the next day. From Linthal, the letter is redirected to a pension in Vaduz, Liechtenstein, where it is received July 7, 1938. Since the Swiss domestic letter rate also applied to Liechtenstein, no postage due was charged at Vaduz.
However, the letter was further redirected to a pension in Mondorf-les-Bains, Luxembourg. There the Mondorf post office charged 60 centimes postage due -- this being the difference between the 20c Swiss domestic letter rate and the UPU rate for a letter from Switzerland to Luxembourg. The Luxembourg postage due stamps show the Mondorf cancel dated July 8, 1938.
Don't confuse redirecting charges with charges for insufficiently paid mail. The latter often incurs a penalty (frequently, double the deficiency); in contrast, no penalty is normally imposed for redirection.