lordy said:
Do yourself a favour and get in touch with the company. If you do not alert them and they realise later they could report you or sue you for theft. It will cost them carriage fees to do a swop so they might swallow the mistake or just ask for an extra compromise payment that you can negotiate down as low as possible.
Please don''t listen to the keep it brigade, they won't pay your fine or do your sentence for you.
Sure thay may get in contact and ask for it back, but sue you? Thats a little over dramatic, its their mistake.
I do however think it couldn't hurt to call the shop. It also couldnt hurt to tell them the box went in the recycling before you noticed