The Central Post Office on Plaza de Armas contains a very nice, free, post office museum (Museo Postal y Telegrafico). It is worth seeing, if only for the somewhat ornate architecture and wrought-iron railings and ceiling. It has very nice displays of post office and telegraph equipment throughout the history of Chile. I found particularly interesting the numerous pull-out, poster-size file drawers containing stamps from a great many countries.
It is definitely worth 15 or 20 minutes when visiting the Plaza de Armas. I enjoyed it, even though I don't speak Spanish.
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