This is especially true for money, checks, and credit cards. Even letters with only photos enclosed have been opened. If you are mailing a number of first class letters or postcards from a poor country hand them directly to a clerk and watch the clerk cancel the stamps. This is because in many countries the price of a first-class stamp is equal to a day's wage so stealing them and reselling stamps is a lucrative sideline for postal workers. Airmail can take anywhere from 7 to 30 days to get to you.
Surface mail can take three months or more.
Before going to the central or main post office, call to see if your package has arrived, what the hours are, how to get there, and if there is a pick-up fee. It may be impossible to establish the phone number, they may not have a clue what you are talking about, and they probably do not care. Some places have lists you check to see if there is anything for you. It all works in the end - patience is the key. Sometimes identification, such as a passport, is required.