```python
from fastapi import FastAPI, Depends
from pydantic import BaseModel
import asyncio

import helpers

app = FastAPI()

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


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


class ChargeRequest(BaseModel):
    amount: float


@app.post("/charges")
async def create_charge(
    charge_request: ChargeRequest,
    billing_client: helpers.BillingClient = Depends(get_billing)
):
    receipt = await asyncio.to_thread(billing_client.charge, charge_request.amount)
    return {"receipt": receipt}, 201
```