Means "I'm done, that's all I have the energy/money,willingness/desire for, there is no more I have to say/add/do."
Importer: "I'm sorry the cap on your bottle of Pickapeppa Sauce was defective. Maybe the distributor can arrange for a replacement."
Consumer: "Thanks but I've shot my wad on this subject." (I'm done, don't want to take it further.)
PS One hopes the provenance of this perhaps somewhat dated colloquialism was not an ugly sexual allusion to the male orgasm (back in the 50's when scumbag was an ugly slang term for condom) but at the time if it was, it did not seem to be generally thought of as such. But looking back, I dunno.