Payment Invoices: Generate and Validate Invoice Line Items with Tax Calculations

Question

## Problem

Build an `Invoice` class that manages line items and computes totals with tax.

**Requirements:**
- `add_item(name, quantity, unit_price)` — add a line item.
- `subtotal()` — sum of all (quantity * unit_price).
- `tax(rate)` — subtotal * rate (rate as a decimal, e.g. 0.08 for 8%).
- `total(rate)` — subtotal + tax.
- `to_dict()` — return structured dict for serialization.

```python
class Invoice:
    def __init__(self, invoice_id: str, customer: str): ...
    def add_item(self, name: str, quantity: int, unit_price: float) -> None: ...
    def subtotal(self) -> float: ...
    def tax(self, rate: float) -> float: ...
    def total(self, rate: float) -> float: ...
    def to_dict(self) -> dict: ...
```

**Example:**
```
inv = Invoice("INV-001", "Acme Corp")
inv.add_item("Widget A", 5, 10.00)
inv.add_item("Widget B", 2, 25.00)

inv.subtotal()    -> 100.0
inv.tax(0.08)     -> 8.0
inv.total(0.08)   -> 108.0
```

## Follow-ups

1. How do you handle rounding (e.g. banker's rounding) to avoid cent-level discrepancies?
2. How would you add support for per-item discounts and invoice-level discount codes?
3. How would you design the `to_dict()` output to be compatible with a standard e-invoicing format (e.g. UBL)?
4. How would you add an immutability constraint so that finalized invoices cannot be modified?

Payment Invoices: Generate and Validate Invoice Line Items with Tax Calculations

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