Make sure you get copies of all your tests!
Thank you both for your comments, I just emailed my endocrinologist with your responses to see what he thinks. I'm sure the POTS is related and I've spent over a decade trying to figure this out and hopefully some day someone will figure it out and I'll get some relief!!
Ask for the Insulin stress test, which will also test your CRH response, which stimulates ACTH.
Unfortunately, testing cortisol is part art and part science. You tested 2 days if I get your testing correctly - the saliva test covered one day and the urine another? That is not a lot of testing. Was it serum cortisol that was low in the morning? Have you had serum cortisol testing as well? Free cortisol can be interfered by medications and other things so it pays to test a lot to see the entire adrenal (were other hormones tested) and see what is up.
Is your sodium and potassium normal? The POTS could be related to an issue with them - were renin and aldosterone tested?