What is the difference between 收了 and 收到了?
我收到了他的信。/ 我收到他的信了。
  I have received his letter.  


Here, 收到了 usually indicates that something I received that was sent by other people actively. I just get it passively. 


  我收了他的钱。
  I took his money. /  I accepted his money.  


Here, 收了 just means I get his money. But you cannot tell whether the money is given by him actively or the money is given by my demand/request.