How to negotiate better pricing on large wholesale orders?

We've been ordering from 2 Korean brands regularly for about a year now. Order sizes have grown from $5K to $20K+ per order. At what point do you typically ask for volume discounts? And how do you approach the conversation? Specifically wondering about: - What discount % is reasonable to ask for? - Do brands prefer larger single orders or consistent monthly orders? - Any other terms worth negotiating (payment terms, exclusivity, co-marketing)?