One of the great features of the Webshopapps Royal Mail extension is that you have control over the rates. You can also specify how many items can go into a letter or large letter. So if you sell say CD's and only one of these can go in a letter then you can manage that, or if you sell elastic bands and 5 can make up a letter you can do that.
To modify use the item from and to fields in the Royal Mail CSV file provided with the extension. By default only 1 item is allowed in either letter or large letter items, otherwise it will default to a packet.