I'd have to agree that the penalty for failing to neuter is a bit short on "teeth," in my opinion. Once they decide the dog is probably healthy, they might decide to take their chances on the warranty.
Like many doodle breeders, I neuter my pets before they go home.
If you are not comfortable with that option, I've seen other breeders charge breeder prices for pups that have not been neutered. This can be hard to collect, of course, and you'd have to be prepared to take legal action, which is time consuming and a hassle.
Take into consideration the breeder pup price as compared to the limits of Small Claims court in your area. You can sue for the Small Claims maximum without hiring a lawyer, and the judgement comes down after a brief court session in which both parties present their case to a judge. Note that you now have a judgement, but have Still not collected any money! If the buyer doesn't pay the judgement, back to court you go...
PS: Document Everything!!! Keep impeccable records.
PPS: I'm not a lawyer, and Am Not offering legal advice!!!