I reach out to customer service. So it turn out there are the requirements for a package to qualify for First Class Package International shipping, which is very weird cause all my default packages for International First Class still work like 5 days ago. But here is the requirements:
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Must weigh 4lb or less
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Package value must be under $400
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Maximum combined length, height, and width is 36 inches (42 inches for rolls/tubes)
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Maximum length is 24"
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Minimum length is 6"
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Minimum height is 4" (NOTE: this means the package cannot be an envelope, it must be a soft pack or box to meet the minimum height requirement)
I have to change my package size to 6x4x4 and it worked. Hope this helps!