[El Gato]

I've been told by folks that should know that there are no countermeasures that can stop the newer MANPADS.

There's always countermeasures. They may not be cost effective, but it's always possible. For example a powerful laser could just burn out the detector on the sensor of the missile. But it's true that decoying the missile seeker, or even just causing it to break lock, is getting more difficult. The missiles won't go after something that doesn't both "look like" a target and act like a target. It's difficult to get false targets to behave in such a manner. Denial (IE blinding the seeker) is more difficult with a passive seeker, although even an Radar seeker can go into a home on jam mode if it's blinded.

One possibility is a towed decoy, which can be made to both maneuver like and "look like" the targeted aircraft. These work well if there's only a single missile going after the target, but if there is more than one, then you need more than one decoy, since the decoy is likely to be killed.

[Valin]

When a plane is 300 feet up or less and landing?

[ProudVet77]

Not to be a contrarian, but no, there are not always countermeasures. Droping a flare is meaningless, most of the newer MANPADs can tell the difference between the IR profile of a flare and the IR profile of an engine. Most MANPADs also have backup detectors. Shine a laser at it, and it will simply activate it's optical (electronic) shutter so as to not burn out the tracking components. And then aim at the source of the laser
The good news is that even a direct hit from a MANPAD may not bring a plane down. Even a dual engine plane like the 767. It may hit one engine, but the odds are it not do a lot of damage to it. The plane will also automatically adjust trim systems to keep it on flight path, as they are programmed to deal with an engine failure.