func postStory(story string) (events.APIGatewayProxyResponse, error) {...}

pt-10.main.go

func postStory(ctx context.Context, story string) (events.APIGatewayProxyResponse, error) {
	err := client.Ping(ctx, readpref.Primary())
	if err != nil {
		return handleError(err)
	}

	var raw Story // <--- this and the folowing two lines was a big gotcha 
	s := strings.NewReader(story) // ^---> because the type of response.Body is `string` and you need it 
	json.NewDecoder(s).Decode(&raw) // ^---> to be what's defined in your schema/json

	result, err := collection.InsertOne(ctx, raw)
	if err != nil {
		return handleError(err)
	}
	/*
	 the following was a big gotcha,
	 cause `result` is type *InsertOneResult
	 and you can't send that as json in your lambda response
	 I had to do some reflection to the result in order to send it.
	*/
	newDbResult = DbResults{nil, result}
	return handleResultSendDbResponse("InsertOneResult")
}