It is what the seller is charging.
I charge £2.05p for a Second Class Large Letter even though the postage cost is only £1.55p
I assume the seller is adding on a small amount for packaging - most sellers use new postage bags or padded envelopes, tape, bubble wrap etc. and they are allowed to pass the cost onto the buyer.
If you don't like what the seller is charging for postage, find a different seller.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)
