Fusion, the seller has abided by eBay's rules which say that you must mail to the buyer's address on the eBay registered account - to do otherwise voids the seller's entitlement to disputes etc. If you wanted the tiles delivered to your mum, you should have emailed the seller well before the sale ended, to confirm whether they would be willing to do this, and get their agreement. Many sellers won't - there have been too many scams in the past in alternative address delivery where the buyer later claims that the stuff didn't arrive, so the seller is left with no goods and has to refund the money.
The rules for sellers are very clear - if they send to an alternative address it's their risk. When did you instruct them with the other address? if it was only on the payment advice, they may not even have seen it. IMO you are not within your rights to kick up a fuss about the delivery charge - the seller has done exactly what eBay expects them to do.