can admin choose shipping method

I'm after an option to be able to choose a specific shipping just for certain products

basically i have the royal mail shipping set up but the lowest charge is £1.39
i sel a couple of things which is just the price of an envelope and stamp ie. 99p
what i'd like to do is set up flat rate for example and set that product to be able to use that method

or be able to lower the royal mail to a lower price for small light items
i've tried dropping the weight to 0.005 but it still shows as £1.39

any ideas please

I am looking for a similar option which the OC basic template does not seem to offer. I know there are extensions that one can buy, but I am not sure they address this issue.

I want to use US Express mail boxes based on quantity ordered. The Flat Rate doesn't allow one to add other rate options in, so that doesn't work. I tried the weight option and it did not calculate correctly. The only thing I have not tried yet is to change the item weights to be higher so maybe it does calculate closer to what the correct cost should be.