There is no diplomatic immunity for military members returning from Afghanistan.  And how would you acquire it?  Very few people go outside the wire.  You would never have the opportunity to be alone and buy anything, assuming you even know who to contact.  Every time we came back from Afghanistan, all our belongings were precleared before we left.  And we went through US Customs when we got back in country.  You obviously never went to Afghaistan, so don't try and act like you know what the process is, because you don't.