Currently, you can only add classes to specific customer invoices by opening each invoice at a time. However, if you haven't turn on the class and location tracking feature, you can go to your Settings so you can add classes to your invoices. Here's how:
Go to the Gear icon at the upper right.
Select Accounts and Settings.
Go to Advanced from the left menu.
On the Categories section, click the Edit (Pencil) icon.
Check the Track classes boxes. You can also check the Track locations box if you want.
Once turned on, you can create categories for them. Here are the steps:
When creating an invoice, you'll have the option to add and choose the class. You can see the screenshot below for your reference.
Please know that we're listening to your idea about adding classes to your multiple invoices, and I'll share this along here on my end. I suggest checking out this link for more product updates: QuickBooks Online Blog. If you have any product-related concerns, you can freely browse them on our Community page.
I'm here anytime you have any other concerns. Have a great day.