What is the difference between 收了 and 收到了? What is the difference between 收了 and 收到了?
1. We say 我收了他的錢
2. But we don't say 我收了他的信, We say我收到他的信了

When do we use 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.