Woocommerce Postage Settings

[wayzgoose]

I set up three Flat Rate postage options: 2nd Class, 1st Class and Express. I added a postage class for bulky items that only go Express by entering 4 on the 2nd class and 3 on the 1st class so that all options showed the same as the Express. The amount I added in the Class is being added to the standard rates of postage. Now when I'm reading up on this, I see that the class is supposed to replace the flat rate setting. Anyone know what going on?
I realised there was a problem when I wanted to add a class for large letter, which would be cheaper than the Flat rates but I can't get it to work using minus.