A post office is a building with post
A post office is a building where mail is collected and sorted, also mail is delivered from the post office. Post Offices also offer post-related services. These are services such as selling stamps, envelopes and packing (and sending) of parcels. The services of a post office are not the same in all countries, there are countries where you can also do financial transactions, or other official papers and transactions. In The Netherlands there are less and fewer post offices but more and more mail agencies (a postal department within a shop like the Bruna or Albert Heijn).