How accurate are home drug tests?

I had been clean for about 5 weeks and then took 3 hits. The next day I got popped with a drug test but I managed to postpone it for 10 days. I took two at home drug tests and passed both. I took the test on Monday and hadn't heard back but today I got notice that I had a certified letter at the post office. I went to pick it up but I was told it would probably be Monday before I could pick it up. I can only assume that it was from the lab since I have never recieved a certified letter before. Just wondering, is it common for labs to send letters or do they usually call. I will be a ball of nerves until I find out for sure but I can only assume the worst. Any comments would be appreciated.