I hate the piece of advice that tells you that you should under promise and over deliver.
To me it says that you don’t trust yourself and that you think your client is stupid.
If you are going to build a business relationship, you should have the confidence to say to your partner that this is what I do, this is what I am going to do, and this is what we can expect. You don’t start the business relationship by hiding actions you are going to take so that you can “over deliver.”