Welcome to my world..............................
As for the enclosed version being more expensive, the cost is based on volume of material. If you have not left an opening, they will assume it is solid. Or it could simply be that you have fairly thick bits. For example, is the belly part hollow, or is the floor level. If it is solid, that is a lot of material. If you haven't done so, think of making it hollow from either the bottom, or by dropping the 'floor' where required.
I am currently looking a different method of providing a better finish to WSF, which will improve the ability of vinyls to stick.