Hello, Edward.

I understand that having the option to add several products and services to one line item will be beneficial for you. Yet, it isn't an option in QuickBooks Online. 

You may consider the suggestions provided by my peer above to add another line item to record several products/services in one invoice.

I included these articles for more details and tips about tracking your inventory in QuickBooks Online.

• Use reports to see your sales and inventory status
• Change product and service item types in QBO

I'm always eager to help if you have any other concerns with your account.