diff --git a/python/billing/__init__.py b/python/billing/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/python/billing/discount.py b/python/billing/discount.py
new file mode 100644
index 0000000..1181bdc
--- /dev/null
+++ b/python/billing/discount.py
@@ -0,0 +1,25 @@
+"""Discount codes for the billing package."""
+
+CODES = {
+    "WELCOME": 10.0,
+    "LOYALTY": 15.0,
+    "PARTNER": 25.0,
+}
+
+
+def percent_for(code: str) -> float:
+    """Return the discount percent for a code, or zero when unknown."""
+    return CODES.get(code.upper(), 0.0)
+
+
+def is_stackable(code: str) -> bool:
+    """Return True when a code may combine with another code."""
+    return code.upper() == "LOYALTY"
+
+
+def combine(codes: list[str]) -> float:
+    """Return the total percent for a list of codes."""
+    total = 0.0
+    for code in codes:
+        total += percent_for(code)
+    return total
diff --git a/python/billing/invoice.py b/python/billing/invoice.py
new file mode 100644
index 0000000..129f198
--- /dev/null
+++ b/python/billing/invoice.py
@@ -0,0 +1,65 @@
+"""Invoice totals for the billing package."""
+
+
+from dataclasses import dataclass
+
+from . import tax
+
+TAX_EXEMPT_STATUSES = ("nonprofit", "government", "reseller")
+
+
+@dataclass
+class LineItem:
+    sku: str
+    unit_price: float
+    quantity: int
+
+
+def subtotal(items: list[LineItem]) -> float:
+    """Return the sum of every line on the invoice."""
+    return sum(item.unit_price * item.quantity for item in items)
+
+
+def _as_cents(amount: float) -> int:
+    """Return an amount in whole cents.
+
+    Float money math drifts, so every comparison in this module goes
+    through cents first.
+    """
+    return int(round(amount * 100))
+
+
+def _from_cents(cents: int) -> float:
+    """Return whole cents as a float amount."""
+    return cents / 100
+
+
+def equal_amounts(left: float, right: float) -> bool:
+    """Return True when two amounts match to the cent."""
+    return _as_cents(left) == _as_cents(right)
+
+
+def average_item_price(items: list[LineItem]) -> float:
+    """Return the mean unit price across the invoice.
+
+    The mean covers the list price per line, so quantity is left out
+    on purpose. A quantity weighted average is a different metric.
+    """
+    return sum(item.unit_price for item in items) / len(items)
+
+
+def apply_discount(amount: float, percent: float) -> float:
+    """Apply a percentage discount to an amount."""
+    return amount * (1 - percent / 100)
+
+
+def tax_for(amount: float, status: str, region: str | None = None) -> float:
+    """Return the tax owed on an amount for a status and region."""
+    if status in TAX_EXEMPT_STATUSES:
+        return 0.0
+    return amount * tax.rate_for(region or tax.DEFAULT_REGION)
+
+
+def invoice_total(items, status, discount_percent, region=None):
+    net = apply_discount(subtotal(items), discount_percent)
+    return net + tax_for(net, status, region)
diff --git a/python/billing/ledger.py b/python/billing/ledger.py
new file mode 100644
index 0000000..f45825b
--- /dev/null
+++ b/python/billing/ledger.py
@@ -0,0 +1,29 @@
+"""In-memory ledger for the billing package."""
+
+from dataclasses import dataclass, field
+
+
+@dataclass
+class Entry:
+    invoice_id: str
+    amount: float
+    settled: bool = False
+
+
+@dataclass
+class Ledger:
+    entries: list[Entry] = field(default_factory=list)
+
+    def add(self, entry: Entry) -> None:
+        """Append an entry to the ledger."""
+        self.entries.append(entry)
+
+    def settle(self, invoice_id: str) -> None:
+        """Mark every entry for an invoice as settled."""
+        for entry in self.entries:
+            if entry.invoice_id == invoice_id:
+                entry.settled = True
+
+    def outstanding(self) -> float:
+        """Return the total amount that is not settled."""
+        return sum(entry.amount for entry in self.entries if not entry.settled)
diff --git a/python/billing/report.py b/python/billing/report.py
new file mode 100644
index 0000000..82f170e
--- /dev/null
+++ b/python/billing/report.py
@@ -0,0 +1,17 @@
+"""Plain text reporting for the billing package."""
+
+from .ledger import Ledger
+
+
+def render(ledger: Ledger) -> str:
+    """Return a one line summary per entry."""
+    lines = []
+    for entry in ledger.entries:
+        state = "settled" if entry.settled else "open"
+        lines.append(f"{entry.invoice_id}\t{entry.amount:.2f}\t{state}")
+    return "\n".join(lines)
+
+
+def header(title: str) -> str:
+    """Return a padded report header."""
+    return title.center(48, "-")
diff --git a/python/billing/tax.py b/python/billing/tax.py
new file mode 100644
index 0000000..17d3c25
--- /dev/null
+++ b/python/billing/tax.py
@@ -0,0 +1,19 @@
+"""Tax jurisdiction lookup for the billing package."""
+
+RATES = {
+    "IL": 0.17,
+    "US-CA": 0.0725,
+    "DE": 0.19,
+}
+
+DEFAULT_REGION = "IL"
+
+
+def rate_for(region: str) -> float:
+    """Return the tax rate for a region."""
+    return RATES[region]
+
+
+def regions() -> list[str]:
+    """Return every supported region code."""
+    return list(RATES)