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...
Thomas Rushton
blog: https://thelonedba.wordpress.com