On page get cart there is a part about "Showing cart in the header?".
It mentions using the &toPlaceholders property with an example to prevent outdated cart info.
However the example doesn't work if one of the tpl properties is set for the snippet.

To make it work with one of the tpl properties you can use the MODX Output Modifier toPlaceholder.
Example:

[[!commerce.get_cart:toPlaceholder=`cart`?
  &tpl=`CartTpl`
  &itemTpl=`CartItemTpl`
  ]]

In the "header" you can the show the cart by adding [[!+cart]]