how can i safely bring an 27 inch imac from us?

If you are bringing it from the US to another country, you should select a carrier that provides international shipping (UPS,FedEX), pack it very, very well, and insure it for the amount you paid for it and hope it arrives in the same condition.

Pack it very well with a lot of bubble wrap and a lot of fragile stickers on the box.

For an iMac, I would place a thin wood or hard plastic panel to cover the entier screen for extra protection.

Double box it (a box inside a box).It makes a much stronger package.

Also clarify the TOTAL cost of shipping with the carrier.

Some (especially UPS) will charge you outrageous custom brokerage fees on top of the shipping fees.

it may differ from country to country but where I live there is a limit as to how much I can import without paying toll

got a huge fine from ordering that macbook pro dock some company produce because it's price was higher then the allowed max purchases

Only your airline can answer if you can bring it as checked luggage or what it will cost to do so. You should compare that cost to what a shipper will charge.

In addition to the other comments concerning packing, I would recommend you use the original packing material if you still have it. The box it came in was designed to ship the iMac long distances safely. If you want to add to the original package then put the original box into something else.

good luck

Cabin luggage? No, it's too big a box for cabin luggage.

As a luggage with all the other luggage? Maybe, but you need to check with your airline first, and I don't know if you ever saw how baggage handlers handle luggage when loading them on a plane, but I would not let them handle my computer!

If your airline will take it, they will probably charge extra for it, like they would for a bike or a surf board, and ask you to cover it with insurance.