We have a Thule. As said upthread, it is surprising how little weight is supposed to be carried, if you follow the car manufacturers recommendations. Mrs B's, with roof rails, is 100kg.
My rule of thumb has always been to try to keep the centre of gravity low. It helps with handling and is a lot kinder on the back and shoulders. So there is a pecking order, depending upon who or what is being carried. Heavier stuff goes; floor wells, spare wheel well, boot, rear seat, roof box. The roof box is easy to pack. You just make sure whatever is in there cannot slide around, either using the internal straps and/or softer stuff. Or if you are short of straps in the carrier, get a couple of self locking straps. My local B & Q had some on sale a couple of years ago for a £1 each. Carrier bags are great as they can be used both for keeping things together, but also when full, for packing around other objects.