Sorry, there’s not much I can do about the parcel postage rates, and I don’t think it could be sent in a letter. I guess another option would be to buy a beach ball and cut the valve out… I’m not sure how good of a valve you could get that way, but it might be worth a try. I couldn’t find similar valves for sale anywhere, which is why I decided to stock them myself.
Alternatively, if you order two valves and some fabric for a seat, it might make the shipping costs more palatable 🙂