Interesting post. I'm a merchandising accountant with one Canada's largest grain companies.... while I don't by any means claim to specialize in contract law, I have learned a few things.
My $2 (because 2 cents don't buy you much anymore) from working in the industry....
I have a customer in say Biggar Saskatchewan, and we come to an agreement to deliver (be it in the current month or a year from now) to thier facility 1,000 tonnes of Malt Barley for $330 / tonne..... their would be no way in hell that we would withhold shipments because we underestimated the cost of truck in our contract bid.
That financial risk is on the seller because we agreed to "delivered" terms of sale.
Likewise, if we sold grain "FCA" which the buyer pays shipping and the grain is loaded at our facility, then we wouldn't accept the buyer backing out of the contract if the cost of shipping suddenly spiked......
The OP is totally in the right to be pissed off.
That being said, if they cost of shipping is nominal, then I would just pay these guys and scorch the reputation.