When your company under promises and over delivers, you will create experiences and memorable moments for your clients that will affect their habits for a lifetime.
Do you remember the last time you received a bit more than you expected?
Maybe you were shopping online for your groceries and when you unpacked your delivery you found a couple of free samples or an extra loaf of bread? There's a good chance that, even though these occasions may be rare, when they do happen you don't forget them. You might even tell your family and friends about it.
Back when I was a travel agent, we didn't have computer generated boarding passes...
(Yes, that is me, sitting at my desk in 1986.
Check out my computer!)
The trick is, knowing what you can and can't do.
Making a promise you can't keep is no way to build a relationship, a business or a career. Only you know what you are capable of, so focus on what you do well. Establish what your client wants and attempt to manage their expectations. Go the extra mile to give them a bit more than they wanted and always set realistic timeframes. Don't over commit yourself and allow yourself some extra time for those unexpected road blocks. This is not manipulation, but rather an honest self-assessment and reasonable communication.
Deliver more than you said you would and you become the hero. At the very least, you should always strive to deliver exactly what you promised.