poŝtoficejo

Two words, one o. Poŝtoficejo is poŝto plus oficejo, and the join eats a vowel. Esperanto compounds by putting the modifying root in front, usually bare: poŝt- plus oficejo. You may keep the linking -o- when it helps pronunciation, but here oficejo already begins with o, so poŝtoficejo is what everyone writes and poŝtooficejo is not. The compounding article works through when the vowel stays and when it goes.

Three roots deep, and all of them yours. Take the second half apart again and the pattern shows: ofic- is a duty or a post in the sense of a job, -ej- is the place suffix that gave you preĝejo, lernejo and kuirejo, so oficejo is the place where the duty is done. Then poŝt- names which duty. Nothing here needed a dictionary.

Poŝto alone often does the job. In speech, Mi iras al la poŝto is ordinary and means exactly the same errand, the way English says going to the post in some dialects and going to the post office in others. Poŝtoficejo is the fuller, more official form: it is what appears on a sign, in an address, and in a timetable of opening hours.

Neighbouring vocabulary. A poŝtmarko is a postage stamp, a poŝtisto the postman, a poŝtkesto the letter box, and a poŝtkarto a postcard. All of them compound on the same first root, which is why learning poŝto was worth more than one word.