Here is the 20 centimes value (Sc #687) from Switzerland's September 5, 1980 set of 4 "Swiss PTT Services" stamps. Stamp publicizes the post giro system and it depicts a postal check ready for mailing and a letterbox.
On October 27, 2019 Ireland issued its Christmas set of six, self-adhesive booklet stamps. One of the four, one euro, domestic letter stamps from this set depicts a young woman posting a Christmas greetings in a pillar box.
This January 9, 1968 20 sentimos value (Sc #982) of the Philippines is from a set of three identically-designed (but with differently colored inscriptions, values and country name) stamps commemorating the opening of the Central Post Office in Makati.