Help With Calculating Percentages & relating numeric values to letters

Im trying to create a Purchase order form, amongst other problems I'm facing, the one I'm trying to tackle at the moment is as follows,

I'm trying to use letters as 'code' for Tax percentages, take a look at the following

Screen Shot 2016-08-30 at 21.13.15.png

so after i have entered the quantity and the unit price, the 'amount' column will populate with a simple
'=[unit price]*[quantity]' when i then enter 'S' into the 'TAX' Column i want it to add 20% onto the 'amount' column

but just to make it that bit trickier i have about three different letters representing different tax percentages

I'm pretty much a noob to excel, I'm pretty literate and capable but simple explanations would help haha

many thanks