I've had post-nasal drip on and off for a number of years, though it's never really been a serious problem for me - more of an irritation, though first thing in a morning when I first get up it does cause me to cough for a short time.
In my late teens I had allergy tests done, as I had mild asthma and allergic perennial rhinitis (i.e. year-round hay-fever symptoms). The tests showed that as well as being allergic to grass pollen, I'm also allergic to various tree pollens, animal dander and fur, house dust, and house dust mites. Over the years I've 'grown out of' the asthma and I'm not so sensitive to the things I'm allergic to.
Over the counter medications for coughs and colds can help with post-nasal drip in the short-term as the epinephrine and pseudo-epinephrine will help to 'dry up' post-nasal drip, but as
@Invisible Woman said, they can cause problems as they act like adrenaline, which can cause problems particularly if you also suffer from POTS.
@Fuzzyhead - The combination of antihistamine and steroid has really helped me with the post-nasal drip. It might be worth investigating getting allergy tested. Your GP should be able to refer you to an allergy testing clinic. The actual test is fairly simple. It can be a bit uncomfortable if you react to any of the allergens, but it's otherwise fairly painless.