Under UCC ยง2-718(1), a liquidated damage clause is enforceable (1) if the amount of damages specified in the clause is a reasonable estimate of the damages; and (2) even if the amount is not a reasonable estimate of the damages, the clause is still enforceable if the amount is reasonable in relation to the actual damages.
To see how the clause in the Rex/Regina example could be reasonable in relation to the actual damages, suppose that someone else also had Regina's idea of selling black-light-illumination hula-hoops. The hoops come on the market just before Rex breaches, and it turns out that they are popular enough to sell for about the same price as ordinary hula-hoops. Thus, if Rex had delivered the 10,000 hula-hoops, Regina would have made about a 5% net profit on her sales of those hula-hoops.