you wont be able to do this using a select list, but you could do it using links.
add another quersytring variable to your links for direction:
<a href="sort.php?sort=retailCost&dir=Descending">Sort by retail cost Descending</a>
<a href="sort.php?sort=retailCost&dir=Ascending">Sort by retail cost Ascending</a>
on the bindings panel click the plus button and select URL Variable, name the URL variable "dir"
in he sort server behavior, select the column that is using the sort URL variable, and click the lightning bolt next to Sort order, select the dir query sting variable.
make sure to un-check the toggle sort on subsequent visits option.