I have the API testers. I don't know if other makes are the same, but the API ones measure both GH and KH in german degrees, and you can convert that to ppm using the equation they give you. If the shop meant that 'below 10' was in ppm, that's as good as you'll get because 1 german deg = 17.9ppm and it's impossible to measure less than 1 deg. But if they meant below 10 degrees, then the test kits will do better.
Hardness testers work differently from ammonia etc. You add drops one at a time, shaking after each drop, till the water changes colour. The number of drops it takes to change colour = the hardness in german degrees. Different makes have different colours, the instructions will tell you what to look for.