In a formula column, the SUM() function allows you to add the contents of several columns together.
SUM(Column1, Column2, Column3, Column4, Column5)
SUM(Amount before tax, VAT)
I have added the Amount before tax column with the VAT column.
SUM(Amount before tax, VAT, 12.90)
In addition to the example 1, I added 12.90 which correspond for example to the unimputable expenses of Colissimo.
NB: In the same logic, I can subtract a sum, simply by adding a negative number:
SUM(Amount before tax, VAT, -10)