```python
from fastapi import FastAPI
import helpers

app = FastAPI()

billing = helpers.BillingClient()  # the shared client instance


def get_billing() -> helpers.BillingClient:
    """FastAPI dependency providing the billing client."""
    return billing


@app.post("/charges", status_code=201)
def create_charge(amount: float):
    receipt_id = billing.charge(amount)
    return {"receipt": receipt_id}
```