I noticed today that the Word Biblical Commentary series has been changed to support dynamic pricing (that is, you'll receive a discount for volumes you already own). It's probably been like that for a few months, but I've only just noticed! Anyway, if like me you purchased the WBC before it became a 59 volume set, you'll be able to purchase the missing volumes. Just go to that page, and the price should be significantly reduced. In my case, I purchased the five volumes I was missing for just under $60.