The basic shipping cost of the item.
Flat shipping: single-item listing: this matches (in)ShippingServiceCost; multi-quantity listing: with a request version of 665 or greater, this is the total of (in)ShippingServiceCost plus ((in)ShippingServiceAdditionalCost times the number of items beyond the first item), and with a request version lower than 665, this is (in)ShippingServiceCost.
Promotional shipping: For a request version lower than 665, promotional shipping has no effect on shipping costs. Otherwise: if the promotional shipping option is lower than other shipping services being offered, the savings is reflected in the returned shipping cost; the shipping service named Promotional Shipping Service (or whatever is the localized name for it) is included among the shipping services; if the promotional shipping cost is lower than the cost of other shipping services being offered, it is presented first in the list (the lowest shipping service cost is always presented first, regardless of whether there is promotional shipping).
See the
Field Index to learn which calls use ShippingServiceCost.