Coinbase API returning "product not found" for valid product ID

I'm using the sandbox API at the moment, and I can query the products, including individually, but if I try and place a buy order, the response I get is { message: 'Product not found' }.

Here's my code:

async function cb_request( method, path, headers = {}, body = ''){

  var apiKey = 'xxxxxxxxxxxxxxxxxxxxxxxxxxx',
      apiSecret = 'xxxxxxxxxxxxxxxxxxxxxxxxxxx',
      apiPass = 'xxxxxxxxxxxxxxxxxxxxxxxxxxx';

  //get unix time in seconds
  var timestamp = Math.floor(Date.now() / 1000);

  // set the request message
  var message = timestamp + method + path + body;

  //create a hexedecimal encoded SHA256 signature of the message
  var key = Buffer.from(apiSecret, 'base64');
  var signature = crypto.createHmac('sha256', key).update(message).digest('base64');

  //create the request options object
  var baseUrl = 'https://api-public.sandbox.pro.coinbase.com';

  headers = Object.assign({},headers,{
      'CB-ACCESS-SIGN': signature,
      'CB-ACCESS-TIMESTAMP': timestamp,
      'CB-ACCESS-KEY': apiKey,
      'CB-ACCESS-PASSPHRASE': apiPass,
      'USER-AGENT': 'request'
  });

  // Logging the headers here to ensure they're sent properly
  console.log(headers);

  var options = {
      baseUrl: baseUrl,
      url: path,
      method: method,
      headers: headers
  };

  return new Promise((resolve,reject)=>{
    request( options, function(err, response, body){
      if (err) reject(err);
      resolve(JSON.parse(response.body));
    });
  });

}

async function main() {
  
  // This queries a product by id (successfully)
  try {
     console.log( await cb_request('GET','/products/BTC-USD') );
  }
  catch(e) {
     console.log(e);
  }

  // Trying to place a buy order here (using the same id as above) returns { message: 'Product not found' }
  var buyParams = {
    'type': 'market',
    'side': 'buy',
    'funds': '100',
    'product_id': 'BTC-USD'
  };

  try {
    var buy = await cb_request('POST','/orders',buyParams);
    console.log(buy);
  }
  catch(e) {
     console.log(e);
  }

}

main();

I've tried sending the params in the body, which responds with invalid signature, even when stringified. I've also tried using the params shown in the API docs, but that responds with product not found too.

Any ideas? TIA

from Coinbase API returning "product not found" for valid product ID