My items are mostly small. I buy packing materials at my local dollar store (regular envelopes, bubble ones, small big, tissue paper, packing tape). I ship in US thru First Class with Delivery confirmation. It is the cheapest I could find. I have not yet had an international order. Figure I will adjust those prices as i get orders. May take a hit or two, but I'll live.
I do use the Flat Rate mailers on occasion. If someone purchases multiple items at once, I might go that route (even if their paid shipping costs are a tad lower).