You also can't scale up the cheap UPU subsidized China Post + USPS shipping. It's for small quantities only. Order ~10+ and they ship via much more expensive means. Try and order a bunch one at a time and customs will catch on and seize them.
The UPU shipped items haven't paid customs fees / duties (or typically have WAY understated the value)..it's an exception for samples/gifts/etc. If they know you're shipping quantities, they want their money.
This is why some items on AliExpress either won't let you order more than quantity 1, or switch to FedEx only shipping if you increase the quantity.
