As a USPS employee that earmarks packages to fly on Airlines and regularly stops packages from being shipped due to hazmat or other issues, I can tell ya, you folks are welcome to ship your spent brass without a worry of regulations using the good ole Postal Service.
My suggestions would be to use the flat rate shipping boxes and make DARN SURE you put the spent brass in a plastic bag, duct tape the plastic bag holding the brass, and THEN put into the flat rate shipping box. Just a simple and cheap suggestion to protect your customer's purchase and your good name, as things DO happen during shipment with any provider.
The weight of the brass, the handling during transport on the airline, and handling during transport via ground transport WILL lead to the box being handled rough, and you need to make sure that brass doesn't come spitting out of the box. That's a lot of weight in a small package, so it's being dropped into the bottom of equipment with a lot of other packages stacked on top of it.
Also, don't want some anti-gun person freak'n out because they don't know that spent brass doesn't have an active primer or gunpowder. Read a post a while back about that, and laughed because I knew what the end result was going to be, an a$$ chewing for delaying someones mail.