additional amount; add-on; supplement
Refers to an extra sum of money, charge, or quantity added to a base amount.
There is a possibility of an additional amount arising in the estimate.
The additional amount from the contract will be billed next month.
追加 is the general noun/verb for 'addition' or 'adding', whereas 追加額 focuses on the monetary or quantitative amount added.
Compound of 追加 (ついか, 'addition') and 額 (がく, 'amount').