Does the post office require that black box above the label, next to the postage amount? This takes quite a bit of ink to print, negating the cost advantage of going to the post office when the cost of an ink cartridge is included! Does anyone know? Couldn't a big X in the area suffice?

USPS is very specific about those labels. That big black box is required for the media mail label. If your label is for a book going first class, you'll notice FCM is printed there instead (no black box).

From the Domestic Mail Manual 102 Elements on the Face of a Mailpiece:

Mailers must print the basic required Package Services subclass marking—"Parcel Post" or "PP," "Media Mail," or "Library Mail"—on each piece claimed at the respective rate. The basic required marking must be placed in the postage area (i.e., printed or produced as part of, or directly below or to the left of, the permit imprint indicia or meter stamp or impression). Optionally, the basic required marking may be printed on the shipping address label as service indicators composed of a service icon and service banner (see Exhibit 3.3, Package Services Indicator Examples):

a. The service icon that will identify all Package Services subclasses will be a 1-inch solid black square. If the service icon is used, it must appear in the upper left corner of the shipping label.