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Which value the Domestic shipping service value are validated against :
When make AddItem call, the Domestic Shipping Service values and other business rules are validated against the Item.Site value you specified in AddItem call. If the Item.Site property is not sent, it is defaulted to the HTTP header X-EBAY-API-SITEID parameter - the eBay site your AddItem API request is routed to and the target site your item is about to be listed. You need to make sure the Item.Site value maps to the numeric X-EBAY-API-SITEID.
For example, if the X-EBAY-API-SITEID is set to 77, then you should define the Item.ShippingDetails.ShippingServiceOptions.ShippingService property, in your AddItem request, to the one of the ShippingService values that obtained from GeteBayDetails API which was made against the site 77.
NOTE. The Item.Site property is always returned in GetItem API and can be used to find out which site the AddItem request was made against.