Answer: when they notice a difference in their business. That of course is hard to gauge when they're not aware if people are going elsewhere for the titles they stock. But is is tempting, tacking-on the "times are tough" surcharge, because the public that has gone through their own postal service woes, will likely accept that little "rounding-up". It's like when, oh, I dunno, a war breaks out anywhere, and the next morning your gas station is charging more. Sellers would do well to get feedback from purchasers that don't complain about their postage rates, but instead just say, "this is the last time because I've found sellers with a fairer postage charge". Perhaps that will stick to their brains, when they're getting so many publicly-visible feedback responses and their sales start to go down.