From everything I can tell, buying a girl a drink should be a "reward" or a positive reinforcement for not activating her ASD (Anti-slut defense) or for allowing you to dance closely and activate the power of your touch..
If she is a 10 or a 9.5 or even some 8 and 9's you will have to neg her no matter what to bring her down to earth/get her head out of the clouds. after you neg her ("ohh thats so cute, did you know your nose wiggles when you talk hahaha", or "there's no need to be shy") if she responds properly by trying to prove to you she's "not shy" or if she starts laughing at your jokes, allowing you to place your hands on her, etc then it is okay to buy her a drink although she shouldn't "tell" you to buy her a drink. you should just say "what are you drinking?"
This is a reward for her and also you want to get her a little tipsy, don't forget. however if she just comes out and says "buy me a drink!" before she has displayed "wantstofuck" syndrome say "get it yourself" and walk away.
or if she is a 10 and you are still wanting to get with her she needs to be severely negged or has friends around you might want to bang also just NEG THE HELL out of her "your nose keeps wiggling when you talk, everyone look! haha"
If she straightens up then you can buy her a drink or otherwise pos her