Sum of multiple columns based on text contents of a header cell

Hello,

I have a spreadsheet of course where I list a bunch of products and their meta data, cost etc.

I then have columns at the end that represent packages and in those columns are select products with the quantity of that product that will be in the package. There are multiple of these "package" columns and they keep getting added on the end. The columns I want to sum up have heading "Qty" standing for the quantity of the product in that row to be included in the package.

I'm looking for a function/formula that will sum the contents (within a range of columns thats expandable as Packages get added) of all the individual rows in my sheet that have the text "Qty" in a specific row which is used as a header row.

The end result would be a column that has the total quantity of parts necessary to make all of the packages.

I can't just sum the individual rows because there is other data in there that would mess up the results.

Any suggestions would be greatly appreciated. If you need more info please let me know.

I've attached the file, it has a lot of columns but many are grouped and collapsed. If you can't scroll horizontally zoom out to confirm the scrolling region is visible.

Thanks!