Items sent in containers still count, so for example a [[backpack]] with 3 [[wood scrap]]s in it would cost you 4 x 10 copper (1x backpack, 3x wood scrap) to send due to being counted as 4 separate items. To be noted is also the fact that combined lumps do not count as one lump. If you have 1 5kg lump you want to send it will be counted as 5 1kg lumps, so combining the lumps will not give you a cheaper mailcost.
However if you wrap the lump and put it in a barrel, you can send 64kg for a 20c postage fee, 128kg for 30c (and so on if Body Strength permits) this . This should work for any combineable item.
=== Delivery times ===