Auto relist items are not included in the free 300 listings, but manual relists are.
I suggest you look again at your listings to find what is being charged, you do not say how much, but 300 free listings a month are adequate for private sellers.
Give item numbers of some items which have been charged, it would be easier to come up with an explanation. I assume you have not used 1 or 3 day listings which ar charge 35p listing fee.