Shipping definitely needs to be accounted for. But might I make a suggestion? Rather than the Flat Rate shipping at $4.xx each, send them via USPS Media Mail for $2.xx each, and buy a case of 100 envelopes from staples, for roughly $0.50 each when bought in bulk. I have run a small ebay store for a few years and have always done it this way to cut costs.
That' possible as well. I run a eBiz now and just flat rate my stuff (it's getting expensive) so thus why I sad that option. How big is the book? We can do the media rate but depending on the size of the book the cost of envelopes may vary.