Oh dear! oh dear! I suspect your blades cost $15 and the within-China postage was $9 making a total of $24. However, you seem to have mis-used the calculator. You need to input your own weights and prices. If I remember correctly, an average card of 200 blades weighs 270g. So you have to tot up your total purchase price (for all items) and put in the total weight (for all items) and the calculator will then tell you about postage and total cost (items plus postage). You pay that much to MisterToa. He buys your stuff and then sends a message asking for second payment for postage from China to you.
I try to clear mu mind of Western expectations about how the site works and then proceed slowly, exploring like a child. Gradually, the site is beginning to make sense to me.
Why can't you ship razor blades? Because the Premier is being elected and Customs won't permit shipment of dangerous goods. However, the reason given to me was that it was not convenient to ship goods. - but it suddenly was again when their new Premier was installed.
I dry-docked several ships in Nantong, near Shanghai but on the other side of a Customs Border. The paint factory and its store of the British paint company was on the Shanghai side, my ships and their needs of paint - big ships, many drums of paint - on the other. It starts out utterly non-sensical but when you get your head into thinking Chinese-style, it's a piece of piss. Learn quick enough and you may retain some hair! Allegedly.