It's good that you remember to provide your valuable feedback to your client though, a bit late (after the deadline) but it's a professional attitude to develop your good relations through admiring and thanksgiving approach.
You are suggested to build good professional and long term relationships with your customers/clients through your best service and best communication.
I agree with you on the importance of leaving feedback for each client, it's something that I always pay close attention to because it seems important to me to give the appreciation that the client deserves.
However, this time I just started a new contract with the same client and that's why I totally forgot to give the feedback from the previous one.
Fortunately, I have been working with them for three years, so I'm pretty sure I'll be able to give my feedback thanks to the solution that Bojan gave me.