What this answers
The average cost of a holding is the total you paid divided by the total quantity you hold. Each purchase is weighted by its quantity, not by how recent it is, so one large purchase moves the average more than three small ones.
Fees sit in one box rather than on every row. Splitting them per row doubles the typing and changes nothing, because only the total reaches the average. Portle keeps fees as their own ledger rows, so the toggle here is off by default.
What this does not answer
A lower average cost does not mean a smaller loss. Buying more shares at a lower price pulls the average down, but the total unrealised gain or loss does not change at the moment you buy. It changes only when the price moves afterwards.
This page does not tell you whether to buy. It reports the arithmetic of the purchases you type in, and nothing else.