不要不要。。。
step one: when she hands you the receipt, smile
step two: flip over receipt, write down your number, prepare cash/card
step three: ask her over, hand it to her(make sure she sees the number) and say call me.smile.leave.
since you like her, you lower yourself for two minutes to get what you want. if she likes you she'll call you. it's a chance.
if you ask her for her number, she'll feel cheap giving it to you. so even if she does want to, she might not.
Forgot to mention...this plan might only work if you look good + she's available.