Shipping costs are calculated based on the cost of shipping from your location to the retailer's customer.
For example, if both you and the retailer are located in the U.S. but his/her customer is located in Spain, the shipping cost would reflect the U.S. to Spain rate.
Similarly, if the retailer was located U.K. but both you and his/her customer were located in the U.S., the shipping cost would reflect the U.S. to U.S. domestic rate.
In general, domestic rates are priced at $5 or less. International rates are priced $15 or less.
For more detailed information on how shipping is calculated and the differences per region go here.