Royal Mail shipping but with fixed prices.

Hello having a llittle issue trying to use the royal mail shipping method, I wish to have two option which seems to be ok and able to do however I wish to set the prices for each and make them fixed rather than been dynamic

any clues how this can be done?

any help would be greatly appreciated.

just noticed i didn't put the version of opencart, is 1.4.7

cheers
Dave