I was researching the same problem and found the answer on another post.

Here is what to do.

In products/services create a new bundle.  Call it “Materials”.  No need to change any of the default settings.

Create your invoice and add this as the first line item.  This creates a space below this line and above line 2.

Add in expense.  Hold your cursor over the four dots one the left side of this line.  Click and drag the line into the space under line 1.  Continue for the rest of your materials.  The item amounts will be totalled and appear as one line on the invoice.