No, you cannot. Just ask, "What is the total?" When you do a transaction, and it involves costs paid to different parties, then you can ask, "What is the cost, overall?" For example, when you buy a house, there are stamp duty, surveyors' charges and agents' commissions on top of the purchase price. Your mother may ask you, "How much does it cost, overall?"

I agree with Ben. You can use "overall" only when there are additional expenses involved. When you buy a new car, you don't only pay the sticker price for the car itself. There are options, extras, you have to register it, get insurance etc. If you told me you bought a new car for $10,000 I may ask you, "And what did it cost you overall?" to which you say, "Altogether, it was almost $12k." At the checkout: "What is the total?", "What's my total?", "How much is it?", "What do I owe you?", "What's the damage?" (informal and trying to be funny).

Yes, that's perfectly acceptable. Any time someone has added up a total price you can use it. Though to me it sounds a bit too formal. If you want to be more informal, you could say "What's the total?"