The best advice is: do nothing!
But if you really have to stop and buy, make sure the place provides degustation, or tasting. Otherwise you are simply walking into your local wine merchant's shop and relying on his advice. And I am not sure tourists always get good advice!
Regional tourist information offices, like the one at Saumur, do have tasting facilities and they should be utilized to the full, though the amount of wine swallowed should, of course, be kept to a minimum.
But wine tasting should not be hurried. So hurry on to your destination and hope you are staying in good wine country!