Formula updating with referenced sheets cells

Hi,

I am trying to work out a shipping calculator.
In one sheet I have the products along with their weights in cell D2
In another sheet (NZPostRates), I have the rates per 5kg bracket also in cell D2 on that sheet

The formula in the total shipping cell (K2) works as follows and is correct for the first row:

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I have 300+ products so was trying to copy the cell formula to all cells in the K column by dragging on the bottom corner of the K2 cell.
This updates the first cells value in the formula as I am after but also updates the cell in NZPostRates by a cell too.
So for the next few columns it changes to:

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etc..

But what I need is for the referenced sheets cell to always stay at D2, like:

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Is there a way to only get the cell values in the current sheet to update and not the other?

Cheers