Yes, it looks like a scam.
Normally, overpayment scams are said to request Western Union transfers from the victim so it cannot be reversed while the original payment to the victim can and is reversed. This leaves the scammer with whatever they paid the victim, plus whatever the victim paid them.
In your case, the original payment is said to be from Western Union. Here are some traps to watch out for:
they ask you to make payments of any kind before paying you: your payment might never arrive;
they paid Western Union from someone else’s account: you will be helping them gain access to someone else’s money; or
they money was in their possession but was acquired illegally: you would be involving yourself in money laundering.
Now, there is a chance that this is legitimate, that they really want to pay you €50 including postage, and have the rest accompany the product as a present to the recipient. But this is unlikely, and you’d be taking a huge risk if you chose to participate in their scheme.
If you are earning €8 gross, your profit margin isn’t a lot. By the time you factor in your inconvenience with money handling etc and perhaps even fuel costs to drive to the post office and back, you might have been better off just throwing the item away - even if the buyer is legitimate.
Paying €42 postage for an €8 item sounds excessive. Adding an additional €50 of messy money opens the way for all sorts of scams, including money laundering.
To maximise whatever profit you get out of the €8 item, you are better off selling to a more straightforward cash buyer, especially if it is a one-off sale.