I have set up products that get cheaper if you buy more, however at checkout the tax is calculated on the highest unit price at any quantity.
1-4 units cost 4500 each.
5-9 units cost 4400.
10 or more cost 4200.
The tax rate is 25%.
The tax calculated at checkout is always 4500 * number of units * 0,25. Thus tax is wrong from 5 units upwards.