Calculate profit/deposit from purchases

[mel1na]

When something is sold, a calculation should take place that returns how much (gross or net?) profit was made and how much deposit was due from the purchase.

[mel1na]

Calculating the taxes due on the purchase could also be possible. Different tax rates for alcohol/softdrinks/etc or a tax rate override for everything must be possible to set.

[mel1na]

Taxes will get a separate issue because the implementation is more complex than the profit/deposit calculation.