This article provides detailed information about Merchant Service Center and how it works with the supported versions of QuickBooks.
The merchant service center can be accessed by going to the company preferences from within your QuickBooks Online.
Sign in to the QuickBooks company file that is linked to a merchant services account.
Click the gear icon in the upper right hand corner of your QuickBooks Online company file.
Select Company Settings to display the preferences page.
Then select Payments. Click Connect to set up Merchant services to the company file or Manage Details if already connected.