if i recall, that was a job for the guys who actually printed my books. i want to say i gave them negatives of my front, spine, and cover art and they fit the bar code in by mysterious means. my typesetter and printer had discussions, though, so maybe they worked those details out between the two of them. my printer, peterson publishing out of ann arbor, mich., preferred negatives as opposed to electronic media. actually, at the time, they charged more for it, but that was because my computer is a PC and they worked off a MAC, as i recall. that was also five or more years ago, so i'm sure things have changed quite a bit.
it's a detail you may not even have to mess with, just be sure to ask the printer beforehand. if you're typesetting it yourself, i'd imagine that putting it on the back cover would be relatively simple. not to sound harsh, but if you can't accomplish putting your bar code on your back cover, maybe typesetting a whole book is a little out of your league right now. my typesetter was an old friend of mine who worked for a newspaper doing such things, and though mine was her first book she said she'd never do it again for what i paid her, which wound-up being around a thousand bucks. then again, i've not checked out any self-publishing programmes, so maybe that's changed a lot, too, because there sure wasn't @#%$ to get excited about back then.
yeah, my experiences are outdated. i'm curious, though, about printers who try to extort extra fees because 'your programme isn't compatible with our computers, so we'll have to typeset it ourselves for a modest $1500.' i wonder if that's ever happened. my suggestion is to be anal: find a printer first and find out what things they'll do for free, like barcode insertion, how much their 'services' cost, if you typeset it yourself will it be a problem, and if you have to use negatives for the cover matter. i searched long and hard for a good printing house, and came up craps, obviously, in all cases until i happened upon peterson. they had the best services, price and quality of the ones i'd looked at. just a thought while sorta on the topic. (they were a week late delivering my books, though, and that pissed me off despite knowing it's probably fairly par for the course for a small-timer like me.)
have you purchased your bar codes yet?