Welcome and thanks for visiting the Community, @dannboisvert. 

In QuickBooks, you can only select one product or service per line item in an invoice. Instead, you can add more lines by clicking the Add lines button below the Service Date column so you can enter several products or services in one invoice. 

You might also want to use bundles to group your products and services. Here's a quick tutorial for your reference: How to Use Bundles: Grouping Products & Services.

I have added our help articles for your future reference: 

• Add inventory products in QuickBooks Online. It helps you learn about adding your goods so you can keep track of your available stocks. 
• Record invoice payments in QuickBooks Online. It also includes steps about recording partial payments in case you'll get paid only part of what is owed. 

Visit again if you have additional concerns, @dannboisvert. We're always around here to provide our utmost help.