Log in to your Checkbook.io account.
Go to "Settings" on the bottom left-hand side of the page.
When in Settings, click on the "Profile Info" tab.
When in Profile Info, look for the "Integrations" section.
In the Integrations tab, check the "Lockbox enabled" box under the Quickbooks logo.
After checking the "Lockbox enabled" box, scroll down the page and click "Update"
It is important to remember to click "Update". If missed, Checkbook Settings would not be altered and the Lockbox would not successfully be enabled for Quickbooks.