Lulz, buying RS gold via mail, eh? Yea, if US all you need is a .41$ stamp and it should be fine. Unless it's a package or big envelop (weighing 1lbs) it should only cost .41$ in US.
unless you're shipping a fucking box it wont weigh any ounces. It'll be a few grams. Think about it honestly. Do 3 bills weigh more than bank statements? No. It's a few grams, 1 stamp will do it.