For more details please do also check my print publication "Holograms on Stamps"

In 1995 the German Postal Service introduced a new stamp packaging product with the  trade name PortoCard®. It is similar to a stamp booklet except that the PortoCard contains individual stamps (ready for use) held in place by cellophane strips. They were first issued as a customer relations and advertisement product for postal service branches and were distributed as give-aways at fairs, press conferences and also as gifts to customers. The PortoCard concept was instantly picked up by private companies and since the Postal Service held the trademark, they began producing these cards for other companies. Shortly thereafter the first PortoCard bearing a hologram was issued. Three more then appeared in 1996. Different PortoCard® types have been issued since then and some of the earliest issues have become hot collectors items since only a few ended up in collector‘s hands.