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Why do it get the error "All locations provided for postage service "50301" are not allowable for this site"?
Although, the code 50301 is the code for the shipping service that you are specifying, the error is not related to
the validity of the shipping service specified.
When you specify a country that is a part of the <ExcludeShippingLocationDetails> obtained from the GeteBayDetails API response in the <ShipToLocation> field in the <InternationalShippingServiceOption> container i.e Item.ShippingDetails.InternationalShippingServiceOption.ShipToLocation , you will get this error.
However if you specify any country from the <ExcludeShippingLocationDetails> (obtained from the GeteBayDetails API response) in the Item.ShipToLocations in the AddItem family
of calls, the call will not fail, but will throw a warning message saying "Some of the ShipTo locations provided are not valid for this site. They have been dropped from your listing"
and drop that country from the list of countries that the Item can be shipped to.
So please make sure you enter the valid <ShipToLocations> value obtained from GeteBayDetails API Response.