Transaction Fee: Calculate the Net Amount After Applying Tiered Transaction Fees

Question

## Problem

A payment platform charges tiered transaction fees. Given a transaction amount and a fee schedule, compute the net amount received after fees.

Fee schedule: a list of `(up_to_amount, fee_rate)` tiers sorted ascending. The fee rate applies to the portion of the amount that falls within that tier. Portions above all tiers use the last rate.

```python
from typing import List, Tuple

def net_amount(amount: float, tiers: List[Tuple[float, float]]) -> float:
    # tiers: [(up_to, rate), ...] e.g. [(100, 0.05), (500, 0.03), (inf, 0.01)]
    # return: amount - total_fee, rounded to 2 decimal places
    pass
```

**Example:**
```
tiers = [(100, 0.05), (500, 0.03), (float('inf'), 0.01)]
amount = 350

First $100:  fee = 100 * 0.05 = $5.00
Next $250:   fee = 250 * 0.03 = $7.50
Total fee = $12.50

Output: 337.50
```

## Follow-ups

1. How would you handle an amount of exactly $0 or a negative amount?
2. What if there is also a flat minimum fee (e.g. always at least $0.30)?
3. How would you build a reverse function: given a desired net amount, what gross amount must be charged?
4. How would you display the fee breakdown (per tier) in a human-readable receipt?

Transaction Fee: Calculate the Net Amount After Applying Tiered Transaction Fees

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