Allso add Luxury, fasion and biological items to the list of items shiped. When the cost and dangers of shiping are within reason, those with power and wealth will try to get more of both. Trade is one of the most usefull ways to do this. War is the other, but tends to be destructive and get people killed in large numbers. Remember the 3I is a -Trade Empire- first and formost.
Spices, grains, dryed and candied fruits, vegtables, meats, hides and fiber products ( wood, animal and plant ), usefull or rare medical item base products, ect. ect. Just think of all the stuff that is shiped out of the south american rain forests. That sells in the richer parts of the world for 10 or even 100 times what it costs to get it to the shiping ports. Things that can be made or grown for less than the cost of production + shiping to the destination world, will be in demand. On top of all the items, limited to one or a few worlds in a sector.
Look back on the history of shiping and you can see things like Empires built on the Ice trade. Where with cheep labor Ice was cut from ponds in the winter and then shiped all over the world at a good profit. This only came to an end when the worlds TL advanced to the point of reliable freezing methods, ( which was supred on by the demand for ice / cooling methods to preserve food and aircondition buildings. ) Yes I know this is an example of a short term low TL trade empire. But it goes to show, that where there are profits to be made, even if only for 20 years or so. Someone will want to make that money.