Penghu certainly deserves its own letterbox hybrid geocache, and what's more letterbox-y than a post office? Even the required stamp is related to the postal service.
The Xiyu Post Office was established on December 10, 1958. Its postal code is 88142. Pretty cool information if you're into stuff like that.
The cache is actually located across the street from the post office, next to the Chunghwa Telecom building. It's in a bottom brick of a partial cinder block wall. Refer to the spoiler photo for more help.
Feel free to use the stamp, but please do not remove the stamp from the cache as it is a requirement for this type of cache. Please bring your own pen.