That's a neat trick, but not something I would have dared try if I couldn't afford the full a la carte price had it gone wrong. I will complain and quibble about prices, but only when I'm confident.

I got something similar by accident recently when I was ordering business cards - usually they charge per run, and I needed two sets, one for me, one for my husband - I filled the form out for one run of 200 (not thinking clearly at the time), and when I went to order said 'Oops, actually we want 100 of each design' - they charged for 1 run of 200 instead of 2 runs of 100, so it was a nice discount. I was expecting, and willing, to pay the higher price though.