royal mail shipping modual help

[koi koi]

hi all
i am using zencart which is great and is up and running for my shop
what i am trying to do is the shipping i have worked it out how to use royal mail 1st and second class post for packets all works great and also corrier rate for if the postage works out cheaper sending it via corrier
my problem is this
some of my parcells i will be able to send as large letter which will be cheaper than packet and i want to pass this on to my customers as i have big compatition but how do i configure large letter items so it does not charge them for packet size if you get what i mean any help please thanks

[koi koi]

anyone help