"My grandmother is at the post office."
Translation:Моя бабуся на пошті.
Unfortunately, you must learn it by heart. Some words require using на and another words require using в/у. We could make a general rule that в is used with words representing places inside buildings (в магазині, у школі, в кімнаті, у церкві, у вежі) and на is used with words representing outdoor places or their surface of the buildings (на стадіоні, на вулиці, на даху, на вежі, на балконі). But as we can see there are exceptions like на пошті.
That's what we did, even as English speakers, with our Ukrainian babas. Once my mom became a grandmother, she was introduced to her grandson as Baba, and eventually she just became Baba to all of us. I saw this same thing happen in other parts of the family. Could this be a diaspora thing? Are you speaking as someone in Ukraine?