If you exchange something, like money, then you may be providing something under an oral contract, even if nothing is signed. And if you introduce a waiver, then you turn that oral contract into a written one.
The point is that it isn't the same service being provided and they are very upfront about that with the customer.
One of the guarantees that probably applies in this situation, especially as it is California which tend to have a few stronger guarantees, is fitness-to-purpose. If there is a service, where an exchange is taking place, there is a guarantee that the product should be considered fit-to-purpose.
> The warranty of fitness for a particular purpose is not limited to sales by a merchant as is the warranty of merchantability. It may be imposed on any seller possessing sufficient skill and judgment to justify the buyer’s reliance. The Code drafters suggest, however, that a nonmerchant seller will only in particular circumstances have that degree of skill and judgment necessary to justify imposing the warranty.” (4 Witkin, Summary of California Law (10th ed. 2005))
The products themselves are unmarked, which means it doesn't matter how upfront that they are with the first customer, because the product is allowed to be resold, and then the limitation on purpose aren't guaranteed to be seen anymore, but it needs to be, if you want to reduce your liability.
And you do get to escape liability with disclaimers quite well. Just look at every tax reporting agency out there. Massive disclaimers about how you are still liable for any mistakes they may make.