I maintain several ecommerce sites as well as own one. I have noticed in the past week or so that tax and shipping costs are absent from some merchants thus not providing a "Total Price" result. Since this is occurring to one of my sites, I thought there may be a problem with the XML feed, but nothing in it has changed nor has G reported any problems. There also seems to be no rhyme or reason as to who is affected, e.g. does not have any reviews.