I have Merril-Edge and TDAmeritrade...and of cours

I have Merril-Edge and TDAmeritrade...and of course TDA charges $6.95 per trade...so my order fills for a lousy 458 shares...and when the balance fills up in the thousands..they TRIED to charge me another $6.95 fee...I only noticed it... because it became a margin call. called it in and they reversed it..got to watch them. NOT sure how Charles operates.