Transaction Fee Calculator — Volume-Based Discounts

Related parts: Start with Parts 1 and 2.

Part 3: Volume-Based Discounts

Adding Discounts

Now we add volume discounts. Merchants who sell more get cheaper fees. You need to count how many successful transactions a merchant has made.

Discount Table:

Configuration Data:

You will get a dictionary with fees for many countries. It looks like this:

country_fees = {
    "ie": {"card": (0.019, 20), "klarna": (0.025, 40), "bank_transfer": (0.006, 0)},
    "de": {"card": (0.025, 25), "klarna": (0.030, 45), "bank_transfer": (0.007, 0)},
    "fr": {"card": (0.027, 28), "klarna": (0.032, 48), "bank_transfer": (0.008, 0)},
    "default": {"card": (0.029, 30), "klarna": (0.035, 50), "bank_transfer": (0.008, 0)}
}

Example Data

Input:

Imagine acct_1 has done 10 transactions already. The 11th transaction (py_11) happens. The 12th transaction (py_12) happens.

Output:

• Transaction 11: Base fee is calculated. Then, apply a 10% discount.
• Transaction 12: Base fee is calculated. Apply a 10% discount.

Math Detail: For py_11 (Ireland Card): Base = 1500 × 0.019 + 20 = 48.5. Discount = 10%. New Fee = 48.5 × 0.9 = 43.65. Round down -> 43.

What You Need To Do

• Keep a count of transactions for every merchant.
• Check the merchant's count before the current transaction to find the discount.
• Only increase the count if the transaction is "payment_completed".
• Process rows in order. The order in the CSV matters.
• Use the provided dictionary for country fees. If the country isn't there, use "default".

Questions to Ask the Interviewer

• Do I apply the discount before rounding or after?
• What if a merchant sells in different countries? Does that count towards the same volume total?
• Does the volume count ever reset (like every month)?