I've imported goods to sell from the USA to the UK. Now, when i want to send these out to customers, what's the best way?

They are packaged in boxes that are approx 12inchesX12inchesX12inches, and weight between 1 and 5 KG's. I need something like a 2 day service.

Two competitors ship for 2.50, and 3.50. However, i test sent a box to myself, 1.6KG's, 12x12x12inches and was charged 6.50 by Royal Mail, and i can't find anything cheaper!

I can't charge people the full postage, or i won't be able to compete with the others, but by charging say, 3 and swallowing the rest myself, it kills the profits, as the profit margin is slim, and relies more on a higher turnover of product!

Please help, is there a better way to send?