Bill, please.
请结账。
Where should I pay?
在哪儿付钱？
It's on me.
我请客。
I'll take care of it.
我来付。（这事我来办。）
What is this for?
这是什么东西的钱？

I'm afraid there is a mistake here.
我觉得这儿算错了。
Let's go Dutch.
我们各付各的吧。
We'd like to pay separately.
我们分开付款。
May I have a receipt, please?
请开张发票。