Best way to conduct AstroMart transaction?

I'm curious what people is the best way in which to conduct an astromart transaction. I'm assuming that a tracking number or delivery confirmation for the seller and a money order for the buyer? Anyone have an opinion of which combination is best...USPS, Fed-Ex, UPS and which form of sending money is the safest? I'd also imagine that "best" could mean with a minimum of time invested, hassle. Just curious to read what sort of arrangements people feel work well and provide a little piece of mind...