Can I send a power bank via Royal Mail or courier service.

No, You cannot send a power bank via Royal Mail or courier service. as they are classed as a lithium battery. http://bit.ly/2LFMKQm

If any company offers to ship for you they are breaking the regulations set by the United Nations and International Air Transport Association (IATA). Amazon.co.uk have been prosecuted http://bit.ly/2LDqNkL