Do royal mail allow bottles to be sent? Should I request that the bottle i buy gets sent via a different method?
No, they dont
It will have to be via a courier/method that carries the goods by a compliant method.
Follow the link above for more information, but more importantly check the prohibited and restricted items lists
However as a buyer the postage method is not necessarily your responsibility. If you specify a method and its not compliant then you have contributed to the problem
If the seller picks the method the reaponsibility remains with them (on a legal basis it's the sender who is responsible if they end up breaching transporting regulations)
Pay using a method that protects you, don't ask the seller to go for the cheapest option - valuables ought to be insured to the right level in case its lost or damaged
If it goes wrong you claim your money back from paypal etc, the sender claims from the courier insurance etc