pre-paid pack always out of stock

Uti Sun courier Empangeni never have stock of pre-paid packs, always saying they don't have, because of stock taking! I loose money because I quote a customer according to the size and amount of box, then they don't have stock, only other way, I have to take bigger boz, and its cost more money, and guess who is loosing that money! If you can't supply don't advertise!!!!!