Within Germany, a letter can weigh up to 500g, after which it is considered a parcel or a packet. A packet (Päckchen) can weigh up to 2kg and costs 3,90. You can save money by sending printed papers by 'Büchersending' and goods as 'Warensendung', but in both cases you must not seal the envelope (use bag clips to stop the contents falling out) and you cannot include any personalised extras, other than a printed invoice/receipt. For more information, log onto deutschepost.de.
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