Billing clerks review purchase orders, sales tickets, hospital records, or charge slips to calculate the total amount due from a customer. They must take into account any applicable discounts, special rates, or credit terms. A billing clerk for a trucking company often needs to consult a rate book to determine shipping costs of machine parts, for example. A hospital's billing clerk may need to contact an insurance company to determine what items will be reimbursed and for how much. In accounting, law, consulting, and similar firms, billing clerks calculate client fees based on the actual time required to perform the task. They keep track of the accumulated hours and dollar amounts to charge to each job, the type of job performed for a customer, and the percentage of work completed.

After billing clerks review all necessary information, they compute the charges, using calculators or computers. They then prepare itemized statements, bills, or invoices used for billing and recordkeeping purposes. In one organization, the clerk might prepare a bill containing the amount due and the date and type of service; in another, the clerk would produce a detailed invoice with codes for all goods and services provided. This latter form might list the items sold, the terms of credit, the date of shipment or the dates services were provided, a salesperson's or doctor's identification, if necessary, and the sales total.

Computers and specialized billing software allow many clerks to calculate charges and prepare bills in one step. Computer packages prompt clerks to enter data from handwritten forms, and to manipulate the necessary entries of quantities, labor, and rates to be charged. Billing clerks verify the entry of information and check for errors before the computer prints the bill. After the bills are printed, billing clerks check them again for accuracy. In offices that are not automated, billing machine operators run off the bill on a billing machine to send to the customer.

In addition to producing invoices, billing clerks may be asked to handle follow-up questions from customers and resolve any discrepancies or errors. And, finally, all changes must be entered in the accounting records.