Depends - in the UK we have VAT (at 20%) but there are exceptions for "non-luxury" goods, such as food. You can't just assume that any VAT / GST will be a flat rate - and for gawds sake don't hard code it as
TotalPrice = BasePrice * 1.06
Because it will change (upwards, usually) and it's a lot easier in the long run to keep a "VAT RATE" column or table and generate the "real value" from that.
For example, if all your goods are VATable, but postage and packing isn't then you need to have two VAT Rates (1.06 and 1.00) and apply them both separately.
Look at your local law - tax departments generally try to help (and mostly fail) but do get extremely upset if you start doing the wrong calculations! See if they have any websites to explain it in "plain language".