Whenever I ordered classic car oem/repro batteries, they always came dry. You had to go out to your local hardware store and buy the acid to fill it. Clearly they didn't want to deal with shipping acid to you. Your best bet is to call your local postmaster for an answer. Bulk acid is shipped around in 55 gallon plastic drums, totes, etc via bulk carriers. I can't imagine anything good coming from the USPS shipping acid packets around the country....unless you can absolutely ensure than the container is so rugged, any type of compression or mishandling could not crush it open.
I wiki'd sulfuric acid in car batteries at 37% solution. The USPS regs you posted stated a 25% limit.