sweetcouple,
It depends on how heavy the package is and the contents. For First Class Airmail, it's an economy way for sending postcard, letter and printed matter. If you are going to send a package, the max. weight is 4lbs, max. length is 24", max. length, height & depth combined is 36". The starting cost is $23.65.
For Priority Mail Int'l service, as I said, if your stuff can fix in the provided flat-rate box (there're 2 sizes of flat-rate boxes - regular and large), there would be a cheaper choice. Max. weight is 20lbs and there would be a 5% discount on the charge if you print out the shipping label online. The starting fee is $17.00.
You may need to check if all your stuff can stucked in a flat-rate box or not and how heavy the package will be before you can decide which option to go for. Of course, the most accurate way to know the cost is to go to the post office and let the worker there to weight the package.
I have once mailed a package back to HK with just a few piece of clothes. I used the flat-rate box and it costed a little bit more than US$10 and the receiver got it in a week.
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