First advice

If you have trouble getting the nerve to talk to girls, get a job in retail. Get a job that requires you to talk to and greet customers and smile. I've spent the last two summers working at a sporting goods store where we have to talk to every customer. Even the bad customers help you defuse tense situations which will be useful. Not suggesting you make a career about working low level retail, but if you want to make some money and improve your social skills go for it.

If I end up working at this place again I want to step my game up and try to do some damage. I'm stuck on how to transition from helping the customer to starting a flirt/pick-up. Thoughts?

Trying to pick up your own customers is even worse than picking up coworkers or trying to pick up employees when you are the customer.

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“Nothing is impossible, the word itself says 'I'm possible'!” ~Audrey Hepburn