I wouldn't say it was illegal, but certainly changing aspects of the transaction you agreed to is not ideal.
Have you messaged the seller to explain you require RM, as this was the service shown on the listing, and this is the service you require ?
If they ignore your message, you need to wait for the item to arrive, and if it arrives late or damaged by careless delivery, turn to eBay's MBG link below to explain the policy to receive a full refund.
https://pages.ebay.co.uk/ebay-money-back-guarantee/
@whirs_s_8