I want to use Power Query to keep track of Inventory

I want to use Power Query to combine about 3000 excel invoices that I generate each month at work and give me total of each item sold. The information that I need for it to combine is on the third worksheet named "Inv" (see attached document) and I can't seem to figure out how to get it to combine the data from it. The first sheet "Invoice" has the same data but also has a lot of merged cells and unnecessary information so I simplified it to 2 Columns on the 3rd sheet "Inv" to just reflect the merchandise and quantities that are entered on the first sheet"Invoice". My goal is to take all 3000 invoices each month and have it add up the quantities for each items so I can see how many we sold and then have it subtract from our in stock count. Are there ways of doing that without using power query? When I add column in power query, is there a specific column formula that I need to enter? All invoices that I want to combine have different names but the format is constant LastName FirstName dd.mm.yyyy.xlsm.