I'm interested in the timing, given the "when you buy this" wording. I can see that you need to overpay all at once, but it seems that overpaying is triggered by buying, so can you resolve other "when buy" effects first? For instance, if I have Haggler in play, may I gain a cheaper card before choosing how much to overpay? Potentially, I will choose to gain a Squire off Haggler, trash it to Watchtower for a Knight, and then I would make my decision regarding overpayment based on whether I want to buy the next Knight. Alternatively, I would trash Squire into Cultist into drawing 3 cards, and make my overpayment decision based on which cards were drawn.