So that people who need a 3-cent button use the service, are impressed by it and become higher margin customers later on.
Is there really any vendor loyalty on a site like AliExpress though? It just doesn't seem that likely to pay off. On the other hand, I assume the amount the vendor is losing on a transaction like this is also minuscule, so maybe it would.
I feel like I'm subconsciously much more loyal to services I have used than I would otherwise like to think. There is so much other stuff going on and I just can't be bothered going through the (probably easy) task of understanding a new service. So I just go back to the service that I've already used, know works, and does amazing things like ship 3 cent buttons for nothing.
But it's not the service you need to be loyal to in order for this to pay off, it's the specific vendor selling that 3 cent button, since they're the one losing money on the transaction.
Most vendors on AliExpress only sell such low-cost items in lots, so you could, for example, only buy these buttons by the dozen. Although that still means you end up with a margin that is ridiculously instead of ludicrously small.