Unable to get postage to increment per item purchased - HELP PLEASE

[Pinkloveshy]

Hi All

I sell board shorts and the issue I have maybe simple to fix, however I am having real difficulty resolving it.
For example a customer add's 1 pair of shorts to the cart postage would be £2. If they add a further pair it should be £4, yet it stays at £2. clearly this could be a real issue.

Any Ideas?

Best regards Gillian
Pinkloveshy