If I use Dynamic DNS such as the free service at https://www.dyndns.com, do I still need to set up static IP and do port forward ?

I have a DSL, most likely with dynamic IP address, and I run a Java servlet to get Paypal IPN messages on my notebook, in order for the messages to reach my notebook, I :

[1] set up static IP and 
[2] did port forwarding.

But I found each time the PC re-starts, it has a different external IP, so I was suggested to

[3] get Dynamic DNS service 

like the free one mentioned above, but now I'm a bit confused, if I have step [3], do I still need to do [1] and [2], isn't step [3] supposed to do [1] and [2] for me ? But since I've already done [1],[2], now I wonder if they would cause trouble for step [3], do I need to undo them ? Or do I need all of them together ?