If you need to change a Google shopping product feed price or enable a special tax, you should rename the custom field.
Conditions - decide for which products the custom field should apply;
Output Value - decide how the output looks like and what will be put into the field. The modification can apply arithmetical operations (eg.: ‘+12’ or ‘+22%)’ on attributes chosen in Output Value column (for numbers only).
After the Custom Field is set up, add it to your feed profile.