International Letter in a Post Box: Rules and Limits
Yes, a post box accepts an international letter as long as the postage is sufficient; short-paid mail returns to your return address marked insufficient postage.
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How to Close a PO Box: Steps, Fees, and What to Expect
Close a PO Box online through your USPS account or in person at the Post Office — either route still needs one visit to collect remaining mail and hand back the keys.