Don't use the USPS. And always remember: you're shipping "sauces."
Laws vary by both state and shipping carrier, but the general rule of thumb is that shipping beer is prohibited. (This excludes special licenses, which you can read about here!) A trader's work-around? Sauce! "If the shipping clerk asks what you're shipping," instructs Haydon, you should say "olive oil or artisanal, homemade sauces or something like that." As long as you're not extremely suspicious, they probably won't give you any trouble.
Also -- skip the US Mail, he suggests. "Traders should avoid using USPS and instead use FedEx or UPS. USPS is more strict and less reliable."
If you want people to like you (and you do), throw in extras
Let's say you live in San Francisco, and set up a trade for Pliny the Elder. It's common in Northern California but relatively rare in NYC, where your trade partner lives. The deal is for the IPA, and the $4$ comparison applies to that bottle. But Haydon says "[it's a]lways a good gesture to throw in extras to round out a trade, especially if you have extra room in your box."