Recycle them, probably. Or whatever they normally do with their vehicles when they're taken out of use for other reasons. It's not like those things live forever.
Oh yes, they have aluminum bodies and are very useful for this reason. You'll also find a few people out there who want them for projects or just to drive around. I knew one person with a VW rabbit diesel swap LLV.
Recycled. USPS's fleet is in dire condition with many vehicles past their useful life and being actually dangerous to operate. Not to mention being extremely expensive to operate because of fuel and maintenance of such old stock.