In the case of check conversion initiated by a billing company: The billing company notifies you that it is using the check conversion process usually with a notification on or in your bill. You write a check for the amount of the payment and send it to the billing company as you normally would. The billing company uses the information provided on the check and creates an electronic withdrawal from your account, which is processed through the Automated Clearing House (ACH) network. This is the same network that is used by many companies to process Direct Deposit of payroll and the Federal government for benefit payments like Social Security.

In the case of check conversion by a retailer:
While at a particular retailer you write a check for the amount of your purchase. The retailer scans the check to endorse it, marks the original check paid, and returns the check to you. This information is then used to create the electronic withdrawal which is processed through the ACH network.

The billing company must notify you that it is using the check conversion process, usually with a note on or in your monthly billing statement. When you mail in your payment, you have authorized that your check payment may be converted to an electronic withdrawal. If you do not want your check payment converted, you should contact the billing company directly.