What does Shantingxia mean?

The word "Shantingxia" is certainly not an idiom.

These four words are not words, but they can be analyzed separately.

Mountain pavilion, summer is two words.

Shanting:

It refers to a small pavilion on the mountain, which is also easy to understand.

Summer:

It's summer now, which naturally reminds people.

Hot weather in summer.

So these two words together mean:

Now in the hot summer, the weather on the mountain is still very hot.

In the small pavilion in the mountains, it is ok if you postpone the delivery.

It means enjoying the cool in the pavilion.