retroactively applying formulas to existing data

I need to apply a 19% discount to purchases of a certain type, from a certain supplier. There are hundreds of entries, and I would like to change this information using a macro instead of tediously going cell-by-cell and changing the information.

Here is an image of my spreadsheet:

excel.png

I need to apply the discount to all purchases with the medium "Book" and the supplier "Dawson". I know I could perform this calculation in an adjacent column, but as you can see, the formatting of my spreadsheet would mean that I'd have to go inserting columns, and there are dozens of sections, before applying the formula, probably with an IF function.

I tried recording a macro by simply altering one of these cells in the way that I would like (multiplying the existing value by 0.81) but it didn't work when I tried to apply it to other cells.

Is there any way to do this?

Brilliant, that worked like a charm!

Thanks
J.

More or less, I can see that you are referencing cells in relation to the active one and specifying their values, then giving a command. I'm not very familiar with coding in this context but I certainly understand the structure of it.

Thanks again, you've really helped me out!