If it's a small order and I've paid for delivery, I'd be pretty pissed off even if the item is more expensive than what I had ordered. It would have to either be useful to me or expensive enough for me to sell on ebay and recoup the post, plus a profit for me to be happy. I mean, if I ordered some screws, as part of a larger order so got free postage but I got a tube of silicone, it would annoy me as I'd have to put another order in for the screws and pay for postage but if they sent me an oscilloscope or fluke instead, I'd be happy.
I shop which RS most of the time anyway, as I have a company account form where I used to work so get free delivery.