To get the applicable RefundOption and ShippingCostPaidByOption values, call GeteBayDetails with DetailName set to ReturnPolicyDetails, and then look for the ReturnPolicyDetails.Refund.RefundOption and ReturnPolicyDetails.ShippingCostPaidBy.ShippingCostPaidByOption fields in the response. The value of the fields can be used in your AddItem request.
The following example can be used as a request to GeteBayDetails using the eBay API explorer.
<?xml version="1.0" encoding="utf-8"?>
<GeteBayDetailsRequest xmlns="urn:ebay:apis:eBLBaseComponents">
<RequesterCredentials>
<eBayAuthToken>[ADD YOU OWN AUTH TOKEN</eBayAuthToken>
</RequesterCredentials>
<DetailName>ReturnPolicyDetails</DetailName>
</GeteBayDetailsRequest>
A typical response will look like the following.
<?xml version="1.0" encoding="UTF-8"?>
<GeteBayDetailsResponse xmlns="urn:ebay:apis:eBLBaseComponents">
<ReturnPolicyDetails>
<Refund>
<RefundOption>MoneyBackOrExchange</RefundOption>
<Description>Money back or exchange (buyer's choice)</Description>
</Refund>
</ReturnPolicyDetails>
<ShippingCostPaidBy>
<ShippingCostPaidByOption>Buyer</ShippingCostPaidByOption>
<Description>Buyer</Description>
</ShippingCostPaidBy>
</ReturnPolicyDetails>
</GeteBayDetailsResponse>