Would you split the tips at the coffee shop? Might make up for any wage difference between the two?
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Do you see a difference between selling at a coffee shop and selling at a cellphone store?
Seems to me a Cellphone store would require direct sales, some sort of sales pitch, trying to get customers to add on things to their accounts or phones...
The coffee shop, on the other hand, seems to be a place people come in mostly already knowing what they want.
You take their order, ask if they want anything else, make their drink and/or food item, and you're done with them and on to the next customer.
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For me... I'd fare better with the coffee shop because of this. I'm not good at selling people things they're not sure they want or need.
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Either way... hope you get the one that fits you best and helps you keep your apartment! 
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fides solus
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LIBRARIES... Hardware stores for the mind