Again, its an offence to knowingly open someone elses post without a reasonable excuse (paraphrase).
An example of a 'reasonable excuse' would be to open the packet to find a return address to send the mail back to. However, in this instance it seems you know that address so just return it to sender.
Well in this case a reasonable excuse would be 'thought it was work related'. A company will normally have something in its conditions of employment allowing them to open any mail delivered to them.
Mind you I would wonder why it was being sent to old work place and could a bill come at some point? If work sent it to the ex employee I suppose they would just send the bill on as well! That could get complicated though.
Most websites such as Amazon and ebay have a separate dispatch and invoice address. And most will only allow payment by card / paypal with a 'registered' address. And this is the address the credit card would be registered to.
So I'd have thought that you wouldn't recieve a bill for the item.