我在美国送外卖,需要找钱么?需要找多少钱?我送上去还是您下来拿?用英语怎么说,礼貌一点日常一点那种.

you are a delivery person. the receipt printed the food price. you have to give change. It is up to the customer to say - keep the change.
if you cannot figure out how much change to give back to your customer, then you should learn math and also get familiar with the local currency first. besides, you should figure out the silvers before reaching the apartment building. Right?
All delivery is to the door. This means you go upstairs right to the customer's apartment. I have never heard of customer has to get downstairs for the food.
Simulated conversation: go up to the intercom of the apartment building. press the correct apartment number. when someone say hello. Suggest you say: delivery from (name of your restaurant). when customer say come in. you should hear a buzz meaning front door unlocked. Open it and go find the apartment. you should tell your customer how much is the cost. When customer hands you the bill, try to make change but slowly. you are hoping he will say keep the change. Or he may say make it twenty or some number. this means he want you to charge him to that number and give him back change from that amount. you can keep the rest for yourself. you should say thank you before you leave.
Are you really in the US? you don't sound convincing.
Never ask your customer - many I give change? it is not polite and sometimes people will get upset. giving you tips is not mandatory. you should not demand it.