If it hadn't been in your possession for 4 hrs and you had reported it lost, the prosecution will have trouble establishing that the drugs were yours, but let's face it, not too many people find someone else's purse, put THEIR drugs in the purse and then they also lose it, to have it later found in the same store that you lost it in.
They would have trouble getting a conviction, but let's face it, We all know what happened and you shouldn't be using drugs, but if you do, at least be smart enough not to carry them where they can so easily be found.
Be grateful the drugs weren't found in your purse during a traffic stop. As far as the employee going through your purse, tough luck. Nothing will happen to her for that. The fact that she did, won't disallow the drugs from being admitted as evidence in court because she was not acting as an agent for the government, but I doubt you'll be charged.
Take this incident as a warning and a wake up call not to use drugs. Life is better when you're not on drugs. Seek help if you can't stop.