Sorted and Filtered cell totals

I am trying to filter cells and find a total value for the items selected.
I need the "SUM" cell to remain at the bottom of my spread sheet and show an updated value for my filtered cells.

I have attached a sample file to show what I am trying to do.

Thanks

Change the formula in C28 to this:

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The autofilter will not hide it and it will only sum the visible cells.