You are right, commercial practices regularly deviate from the law, either by explicit agreement or one party imposing its will on the other. Neither of these changes the law when the contract is otherwise silent, especially between a vendor and a lone one time buyer, in the cottage industry context or otherwise
Unlike you, I have never accepted any cost arising from a vendor's failure to supply what they promised to me, perhaps because I know I don't have to. As many have pointed out here, a simple email or phone call tends to remedy these type of things.
But sometimes they are not remedied and for those that ultimately find their way into a court room, usually where the cost of the return is very significant, as a bare skimming of case law will confirm, the shipping costs are those of the breaching party.
Big people (usually companies) short shrift little people (usually buyers) everyday. That doesn't make it legal or right. It is this fact that is the impetus behind consumer protection legislation, where, because vendors are normally more powerful and resourceful than the consumer, the consumer get even more rights than they would in the common law.
In Yang's case, the common law is a sufficient remedy for him. Consumer protection legislation may give him even more, depending on the states involved.
Harald, stripped of the insults and innuendo, your point is a good one. But its anecdotal, and fortunately 500 hundred years of common law is based on a bigger universe that the limits of your commercial life experience.