Found on a stamp vending machine:

OK, it’s not hard to figure out from context that “semipostal” simply refers to the fact that some of the cost of the stamp goes to the Post Office, while some of the cost goes to the chosen cause.

But given the prevalance of the term, “going postal” as associated with violence, perhaps using the label with the “Stop Family Violence” stamp was not the best choice. Now, I’ll apologize in advance for the gallows humor here, but would “going semipostal” be what happens when you want to go to work and shoot people but decide to stay home instead?