The service best suited for this requirement is AWS Budgets, since it enables organizations to create custom budgets to track their spending and usage. It also offers notification features when expenditures are close to or exceed the budgeted amount. Although the AWS Cost Explorer helps in analyzing spending patterns and visualizing usage costs over time, it does not send budget alerts. AWS Billing Conductor is more related to customizing how billing data is presented and shared, rather than monitoring budgets and alerting. AWS Trusted Advisor provides recommendations for optimizing AWS services but does not alert based on cost thresholds.