差額 and 加算: a price difference and an added charge
Compare the difference between two prices with adding a charge to a base amount, keeping a separate amount distinct from the final amount.
The difference between two amounts
差額, read さがく, means a difference in amounts. 二つの商品の差額は千円だった。 reports a one-thousand-yen difference between two products.
For products priced at four thousand and five thousand yen, the difference is one thousand. That is not the entire price of the five-thousand-yen product.
Adding to a base amount
加算, read かさん, means addition. 基本料金に配送料を加算した。 reports adding a shipping fee to the basic charge.
With 基本料金は四千円、配送料は五百円です。, adding those two items gives 4,500 yen. Distinguish this from the five-hundred-yen shipping fee alone.
A change payment and the full price
四千円の商品を五千円の商品に変更し、差額の千円を払いました。 reports changing a four-thousand-yen product to a five-thousand-yen product and paying the one-thousand-yen difference.
The additional payment and the new product’s full price cover different scopes. For 加算, identify what is added to what rather than replacing the final amount with the added amount.
In the stated change from a four-thousand-yen product to a five-thousand-yen one, how much extra is paid as 差額?
Show the answer and why
One thousand yen. Five thousand is the new product’s price; the difference between the two prices is one thousand.