If you carry water to a store to be tested for TA, be SURE that you (1) fill the bottle 100%, and leave NO air space, and (2) keep it cool. Otherwise, you'll get a bogus result.

Chlorine, pH, and TA results are 'perishable' and will not be reliable if (a) the sample is improperly collected (air space, mainly), (b) mishandled (shaken, vibrated, etc.), (c) overheated, or (d) stored too long -- 6 hours max, mainly for chlorine.

Actually, if there's oxidizable goo in your pool -- likely during startup -- you'd almost have to 'race' the sample to the store to get a result as accurate as your home test. And if you half fill a bottle and then agitate it (rolling around on the floor, driving a rough road, etc.) the sample will be read higher in pH and lower in TA than is actually the case in your pool.