So, RNG boxes suck.
I would much rather see OCP items added to an NPC with a fixed Silvervine Fruit cost.
You already have ranks for drop rates, you could price items accordingly to that. A silvervine fruit is approximately 10c USD, so can use that as a baseline.
By making it cost fruit, you could bypass billing restrictions.
People could then fork out money for something they want, rather than gambler's remorse.