It depends (example provided) on the month.
IWM has this neat thing going where people can nominate minis to be pulled from the archive and sold without the fee.
The list gets voted on and the top 4 winners are put in the
Archive Special return section of their site.
Those minis in that section can be purchased without dealing with the archive fee, but it only lasts the month that they won in. So currently in this example the Hellspawn HSN-7D is in this section. I can purchase without dealing with that fee, but when the new list of minis gets voted on and we have the new winners the Hellspawn HSN-7D is then put back into archive and any purchases I make during that time deal with the fee.
If I want that mech without the fee again, I have to nominate and then hopefully it gets on the list and enough votes to win.
I hope that helps.
Dragon41673 though is right about the archive fee's initial charges. The $8 is tacked on the first mini, but others of that design don't have to pay extra.
So if I wanted a mini that was archived it is best to buy multiples of that design because I'll only get the extra $8 charge on the first mini of that design.