Gillian Cropp, a San Francisco medical researcher, ordered a new cell phone from Nextel. When it arrived in the mail the other day, she discovered that she owed $74.99 for the handset (which she expected) and $19. 12 in sales tax (which she did not). That's a nearly 25 percent tax. Or is it? Cropp learned -- after wading through a series of Nextel service reps - - that despite paying a discounted amount of $74.99 for her phone, she was being taxed for the more than $200 full retail price. "I was incensed," she said. "If you get a...