Did the store use a digital tester? Those things have their place but they are for sure not the best way to test a battery, especially one that is having issues.

The battery could have a bad cell and when you driving it enough its proping the voltage up enough to start the car OK, after it sits a day or two that is no longer the case and it cranks slow. Low cranking voltage is very hard on electronics so try to track it down ASAP of course. You might go by an Interstate Battery Distributor and ask them to do a load test for you. If you can carry it in to one of them they can keep it, charge it, and properly test it to make %100 sure its good. We do that at no charge here, I am not sure about others.