S.C.
The 99.9% accuracy claim applies to positives. There's either a detectable amount of the pregnancy hormone in your urine or there isn't. So if the test is positive, you ARE pregnant...no question.
Negatives, however, can be false. Different women produce different amounts of the hormone at different times of the pregnancy. My SIL was never able to tell from a home test if she was pregnant. They were always negative (2+ weeks past her missed period), even when she later confirmed the pregnancy with her doctor. For me, they worked just fine the minute I was due.
So again, there are no false positives, but there can be false negatives. Go forward with the knowledge that you could indeed be pregnant and take care of yourself accordingly. OB's don't usually schedule the first pregnancy appt until 10 weeks (counting from the first day of your last period), so I would just wait until then to pay for the visit.
-S.