Delivery costs aren’t even easy if you use actual weight rather than percentages, because weight doesn’t take into account bulk and some shopping carts are not sophisticated enough to figure bulk into the equation, which means a shop may be forced to settle on, say, small parcel rates for shipping rather than allow for large letter.
I will briefly comment that since moving to Woocommerce, I have at last been able to differentiate between definitely bulky items (paints, larger figures) and items that will go into large letter packaging, but I still have to decide the break point on weight that triggers an order going by large letter or small parcel if it is not a bulky item (items with the shipping class “bulky” automatically trigger small parcel rate regardless of weight). Getting it right is worse for international orders.
As Martin of Vexillia says above, there are all sorts of hidden costs to take into account. It costs me £2 to drive to a post office, plus vehicle servicing costs for the year, so what’s the add-on for that for just one parcel or 20 parcels? Nowadays, of course, I can just book collect and fix extra costs at 72p per parcel.
I note that Perry charges typically £7.50 per pack and 12% UK postage, so assume it must be them (12% is an unusual amount to add because it’s not as easy to work out as 10%). So you can just accept you’re being lured in with cheap figure prices and that there’s a cost for that. Or that Perry Miniatures has calculated that its average UK delivery costs work out at 12% and so that’s what it charges for everything.