IIF Statements and Update Queries.

[nicknicknick]

Not sure where this belongs, I'll try here as I believe it is for queries.
I have several problems.
I am making a pretty basic database for a 'fake' pet store.

First off, I need an IIF Statement for GST.
Some of my animals are taxable, others aren't, so I need an IIF Statement 'If Taxable=True, Apply GST, else Don't'

Also, there is to be a discount of %20 on the postage to 'VIP' Customers.
So I also need an IIF Statement,
'IIF VIP=True, 20% discount of postage, else don't'

Thirdly and finally, I have a problem, which is to do with the Sales and Stock.
When a sale is made of say 4 cows, I want the Number Of Animals In Stock field to decrease by 4 (that is in the Stock Table).

Thanks.

[pbaldy]

This type of thing:

IIf(Taxable=True, GST, 0)

or

IIf(Taxable=True, Price * GST, 0)