Apply percentage fee based on the post-discount order amount, instead of the list prices

I think for that it wouldn't be possible with the "Percentage of order subtotal" plugin. But we could potentially support that with the OrderItemPercentage ("Percentage of each matching product added to the order total") plugin.

We could do this by applying the fee on the top of the adjusted total, like the following:

So changing this:

    $adjustment_amount = $order_item->getTotalPrice()->multiply($percentage);

to:

    $adjustment_amount = $order_item->getAdjustedTotalPrice(['promotion'])->multiply($percentage);

Though perhaps we should make this configurable... Not sure.

Maybe this can be turned into a feature request instead.