```python
from fastapi import FastAPI, Depends, Request
from fastapi.responses import JSONResponse

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, client: helpers.BillingClient = Depends(get_billing)):
    data = await request.json()
    amount = data["amount"]
    receipt = client.charge(amount)
    return JSONResponse(content={"receipt": receipt}, status_code=201)
```