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UK flooded with forged stamps despite using barcodes — to prevent just that
(www.bleepingcomputer.com)
This is a most excellent place for technology news and articles.
Here in NL, you can get a 9 letter/digit code from the app or website to write (or print) on the envelope. For example:
A 8 F
2 C D
5 P V
Ay one point Deutsche POST even had a sms-service for stamps. Probably thr easiest way to get a stamp.