On the subject of taxes, however, at least in the UK, I would rather store the price next to item, not at the transaction total. Not all items are taxable in the UK, so storing the value (before tax) and then the tax value makes more sense. Say, for example, you purchased a Book for £10.00 and a lamp at £20.00, the total tax would be £5.00. Without it being applied to the items you wouldn't know which item(s) it applied to. However, if I said that the Book was £10.00 with £0.00 Tax (as books are tax exempt), and the Lamp was £20.00 with £5.00 tax, this makes things easier.
And, for extra design fun, there are multiple tax rates, depending on what's being sold... So we can't just store "yes, this line has VAT", but we need to know which level / tier of tax is applicable...
And then, just to add further matters, some areas of the UK are tax exempt! For example, if the risk is based in the Channel Islands, or the Isle of Man, then you don't pay IPT (notice I said the risk; so if you live in the Isle of Man but you're insuring a house in Suffolk, you have to pay IPT!).
Thom~
Excuse my typos and sometimes awful grammar. My fingers work faster than my brain does.
Larnu.uk