Fees explained

Hello guys,

I've purchased some QDVE stocks (I'm located in Europe). 1st time I've purchased 3 shares at 32.25E and the tax was 1E/share (i find this extremaly high ... like 3%). I had fixed fees set by default. I've then changed to tiered and purchased another 3 shares at 31.645E/share and this time the fee was 0.425E/share (less than half). How are these fees calculated ? Are there any intervals for them ? I'm asking because maybe it will be cheaper to buy 10 shares or 20 shares at a time instead of 3.