Wikipedia says: As of 2016, the Mi-26 still holds the Fédération Aéronautique Internationale world record for the greatest mass lifted by a helicopter to 2,000 metres (6,562 ft) – 56,768.8 kilograms (125,000 lb) on a flight in 1982.

That is 56.8 to 62.5 tons, depending on the type of ton. Lift two containers at once.

Making things even easier, the shipping company knows the position and weight of every container. The containers have been carefully placed, keeping the weight low and spread evenly. The containers on top are the low-weight ones.

Two paragraphs earlier, Wikipedia states "the Mi-26 has a payload of up to 20 tonnes (44,000 lb)". Exceeding the documented maximum payload by almost a factor of 3 sounds like an excessive amount of overengineering, even by software standards. :-)

You're right that most of the heaviest containers would be at the bottom of the stack. That's kind of a mixed blessing though..

That's at the max takeoff weight (56,000kg), including the weight of the helicopter, fuel, crew, etc.

Depending on the temperature as the site, that could be a lot lower.