Thanks, and that's what I originally thought: the transaction breakdown is label £3.38, VAT 20% 0.68 = £4.06.
But I checked previous labels that cost £3.38, and the breakdown is: label £2.82, VAT 0.56 = £3.38.
£3.38 is usually Royal Mail 2nd class small parcel. However, the label in question is Evri and has '84 AIR Parcelshop' on it. So I assume the buyer managed to select a different delivery option, and eBay has charged me the excess?