```python
import asyncio
from fastapi import FastAPI, Depends, Request
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")
async def create_charge(
    request: Request,
    billing: helpers.BillingClient = Depends(get_billing)
):
    data = await request.json()
    amount = data["amount"]
    receipt = await asyncio.to_thread(billing.charge, amount)
    return {"receipt": receipt}, 201
```