13 oz. is the magic number for being able to use US Postal Service First Class Package service, which is usually the cheapest shipping option. Beyond that it becomes a hunt between UPS, Fedex and the flat-rate Priority Mail options from USPS.
One approach you could take is to attempt to keep a few heavier, ultra-stable known-good references at home, and then ship around a lighter transfer standard, which can be calibrated / adjusted to your known-good references at home after each leg of the trip. This is what I'm going to attempt to do with resistance standards, anyway (oil-immersed at home, copper-wire tempco resistor compensated for the traveling references).