What I do is task those single bolts to be brought to my Trade Depot. Using DFHack, I can hit shift-enter to select everything in my Depot, and then I sell it to the next caravan to buy more bolts from them. I haven't bothered to figure out whether or not that's cost effective, because the more transactions I make, the higher my negotiator skill goes up, the better bargaining rewards I manage.
I thought there was a limit on how much the trader skill could go up from each caravan. And that it's tied to dwarf money, not number of transactions.
And for the most part, I can only get them to accept trades where their profit is roughly half the value of the things and this barely ever changes, even when they're ecstatic.