It should be about $3 or less.
It costs more than that to ship a pack of guitar strings (I work in music retail shipping equipment all across the US). USPS is the cheapest and that costs $3.00 plus $0.95 for insurance. FedEx and UPS come in at around $11 to ship (including insurance) halfway across the country, and that's at our discounted rates. I literally lose money every time someone orders a pack of strings through me.
Companies that ship stuff like that for free are using them as loss leaders in hopes that you'll come back and buy something big from them that they can recoup the shipping costs and make a profit. I would bet that USB PacMan light comes in at around 10-15 pts of margin, so once they pay the credit card fees, if you weren't shipping it, they'd be making a whopping $5 gross profit. Again, that's gross profit. Even if they have a whole bunch of robots that handle literally every step they can, net profit on a piece like that (after you pay shipping) can't be much more than $2, and I'm probably guessing high.