I had a dude do that to me a few years back. I was trading him my modded Toby Pro for his Lakland 55-01. I was so excited to get my new-to-me bass but I was extremely PO'd when I opened the box to find my bass. I guess he had a change of heart during the shipping process. I'm cool now but I would have been a whole lot cooler if he called, emailed, or even PM'ed me about it. I got NO reimbursement for the shipping costs either.
I'd say if you do decide to cancel out the trade, make sure HE KNOWS ABOUT IT and at least offer to reimburse shipping costs. This was your decision to back out so it kind of falls on you to take the responsibility for shipping, IMO at least.
That is definitely not the way to do it. Nobody wants a surprise when you open a package. And no reimbursement on top of that is completely lame.
Some people don't want to deal with communications when it's uncomfortable. But that's something that adults have to deal with from time to time.

