I don't think the price of those small shipments from China is necessarily crazy.
Looking at flights for people: you can get a return flight to China from the UK for less than £350, which means less than £175 one way.
Let's say that's about 100kg (person plus luggage) then this means that flying 100g from China to the UK costs less than 17.5p or 20c in euros.
I expect that cargo will be cheaper than passenger plane, so shipping small stuff from China to Europe can indeed be very cheap. It's about volume and efficiency.
Air freight rates vary wildly by demand/bulk discounts/route/etc but $2-8/kg is typical, with a median around $4/kg. Commercial flights are often cheaper, and especially when you consider the weight of seats/water/food/crew/etc.
The plane itself isn’t the only cost of shipping. Even sending a letter is more expensive, and there’s no plane involved. You have to pick it up, bring it to the airport, and deliver it to the door from the destination airport.
Obviously. This is just to illustrate that what the "real cost" is is not as obvious as what some comments suggest and that it is likely much lower.