Want to buy an iPhone in USA?

I have got a relative living in the US. Can he order an iphone from apple store online with some carrier (not unlocked one) and get it to my country? It turns out to be a lot cheaper. For eg: If he orders an iphone 4s with AT&T carrier, it will cost around $99 (for the phone) + $50 (approx. For activation and such).So maximum it will cost around $150-160. I can unlock the device here for very very less price. My relative can select some PAY-PER-USE plan but he won't use it at all in USA, n so he won't be charged of it. Is that okay? Will it be considered in any way as illegal?

To get it for that price you have to purchase with a plan. If you don't it will cost much more from the store.id personally go onto ebay or craigslist and have him buy you one and ship it down.

You might be able to get it unlocked in your country, but your relative is USA has to still keep paying the bill for the contract he signed. When you buy phones on contract in USA, you have to promise them that you will pay them a specific amount of money every month, whether you use the phone or not. The price over the course of 2 yrs. (the contract is worth 2 yrs.) adds upto about $1500. Which is very expensive. Please buy unlocked.

You did neglect the problem about the contract. You'd still have a 2 year contract to deal with. Also you have to wait 2 weeks to cancel the contract & you would have to pay $325 USD.
Again, also, you wouldn't be able to use a "pay-per-use" plan (actually called pre-paid/pay as you go) since AT&T does not offer those for the iPhone & even if they did it would cost more than the $99.