Had similar last year with a cheap Blu-ray off eBay, hadn't arrived after two weeks so contacted the seller who immediately refunded which I thought was a bit odd, a few days later had a card through the door saying I needed to pay a fee so went down to the parcel office to see what it was and it was a Blu-ray sized envelope with a fake stamp label like that on it. Didn't pay it and told them to send it back, guy there said ita been happening for a while which is why they brought in the new stamps with the barcodes.
It can be a genuine mistake from the seller though, spoke to a school this year and they had bought a load of stamps from Amazon, which they said they normally do And had no issues, but ended up being scammed and sent a load of fakes which then caused all the parents to have to pay a fee when they sent out a batch of letters with them.