Receipts and Invoices are two very distinct terms. Invoices are used to charge clients, for instance, for HVAC system setup or thermostat repair. Receipts, however, are used to acknowledge a payment transaction received by a customer to your HVAC business. An invoice is a request for payment. It specifies which services and components a customer must pay for, the expense of each service, the overall sum due to be paid, the payment due date, the terms and conditions of payment, and customer details. In the billing process, a receipt serves as proof of payment. Clients receive a receipt after paying a bill or invoice that details the total value paid, the date of payment received, and the mode of payment utilized.