Should I complain to seller about this scenario?
I bought a Wacky Mini off eBay, and the seller charged $3.95 for shipping. Instead of going to the post office and shipping it with proper postage, the seller just put it in a plain envelope and stuck two $0.55 stamps on it. So when it arrived, I had to pay $2.90 in postage due to get it because the envelope was bulging. So I feel like I paid almost $7 to ship a $1 mini. I definitely won't buy from that seller again.
Here, the seller obviously knows the rates, and is charging you the correct price for 1st class parcel and is then turning around and trying to beat you or USPS by playing "dumb." You paid the full postage & the seller tried to game the system. IMO the seller is totally responsible, but as someone else mentioned, how much of your effort do you want to put in to recouping your loss.
You could email ebay or threaten to (or just) leave negative feedback. Maybe if the seller sees you got stuck with the bill & potentially gets called out with negative feedback he may issue you a refund. Technically, he has the money to do so since he underpaid the USPS what was owed and you paid the full amount.
BTW, did you message the seller about the issue and ask him to make it right?