You'll make way more money in the long run if you own up to mistakes like this.

It builds trust in your company, so that when they think about renewing their contract they remember you have integrity and aren't trying to rip them off. Instead, you believe in your product and are actually trying to sell it on its merits.

It also becomes a funny story they tell their friends / others in the industry, which could lead to further business.

There are no downsides to owning up (I'd even claim the upsides above), but there are some possibly gigantic downsides to not owning up.