Seeds, and other biological derivatives.
Minerals and ores are likely to be found anywhere, mined off of other planets or asteroids or the like - and perhaps elements to make up whatever alloys or compounds are needed for manufacturing, or living. After all, elements from the periodic table are unlikely to change if they're found on an asteroid or a new planet.
However, biological elements are probably going to continue to be a limiting factor. The places they can be obtained are limited, dependent on a lot of other limiting factors, and they will be in high demand (because they're limited) and thus quite valuable.
So, people want (for example) bread. Wheat bread, finely ground white flour bread. They might not be willing to pay the prices for loaves of bread to be shipped, unless they're really, really rich, and even bags of flour or grain are likely to be too expensive, too much spoilage in transit, to be economic to ship around - but some corporation or other ships wheat seeds, and sets up a farm/hydroponics/whatever, and starts growing wheat to be processed locally, and the price per loaf drops to something hefty but not unreasonable, because people would want that constancy of foods from home, or exoticism of foods discovered from other planets. Or their familiar vegetables and pantry staples.
And herbs and spices are a whole new level of good tradables - they would be in very high demand (a small amount brings familiar flavors), and pretty low bulk - either as seeds or finished products. It's one of the reasons spices were so prized for trade goods historically - when this situation was more or less fact, each civilization was self sufficient and travel was long, costly, and unreliable - so luxury goods were shipped, not so much raw materials.
Beyond foodstuffs, many plants are used for other things - cloth or fiber (cotton, linen, hemp), dyes, medicines, drugs (both medicinal and recreational) the many uses of plant matter (paper, wood for fuel or building, cardboard, charcoal), chemicals used in manufacturing processes or plants used for environmental effects (ornamental, or to crowd out other plants, or to provide some resource). Sending the finished products might be possible, depending on supply, demand, and price - but sending the means for each colony to make their own independently (seeds, manufacturing tools and equipment, etc) would likely be cost-efficient and sustainable enough to make it economic to ship initial loads, replacements for crop failures, occasional followup loads of seeds to increase diversity, and so on.
So it need not be a one-time cargo, either. Many of the products people use interfere with the plant's reproduction (wheat that goes to bread does not go to the next wheat crop, many vegetables are picked and consumed before the seeds finish maturing). Even with a concerted effort to make the populations of plants on each world self-propagating, periodic infusions of new diversity from fresh seeds, or replacing generations lost after crop failures, would probably be quite welcome and very valuable in their own right.
A load of mixed seeds would be pretty valuable, in high demand, and still give enough yield for the space they take for the end results to be valuable but not out of reach for people to actually buy. Even if the long shipping, imperfect storage, etc leads to some amount of loss or spoilage, some seeds would be unexpectedly valuable (from crop failures, or a newly discovered use) to make up for it.
Animals and their products would also be in high demand. Again, shipping adults would likely be too much expense for too little return, too much lost in transit, too messy. Only the really rich could afford such. But shipping genetic material for cloning, or for in-vitro fertilization, might make the end products reasonably profitable. Somebody pays the really high price for a ships-load silk from earth, somebody pays the same price for carefully preserved silkworm eggs (or spiders eggs, for spider silk instead) - and the silk they produce locally is a very popular and more affordable luxury item. So the local herds (sheep, cows, horses, the whatever from colony x and the whatsits from colony 3a) might have to be shipped over for high prices and lots of loss to begin with, depending on the tech levels involved (though maybe even cloned beasts might be enough to start a herd, if only to the second generation), but they could be supplemented with new genetic material from in-vitro fertilization within a reasonable level of technology.
Manufacturing equipment is probably also going to be popular - raw materials can be found, sure, but tools (and further back, the tools to make the tools) are going to be something that's a lot easier and cheaper for a central, specialized manufacturing place to make in bulk than individual colonies. The price for making everything themselves from scratch is effort, especially when a colony is just starting out and having to do everything for themselves. The price of shipping would offset by the production capability, the further back along the process you go - so a shipment of tractors, gets paid for in how much more can be produced with them than without them doing everything by hand - the pieces to make a tractor factory, gets paid out in every tractor and everything every tractor made can produce. The less that can be shipped, and the greater its impact, the more likely it would be cost-effective. And, there would be a continuing trade in repair parts and upgrades.
It might take a while for the colony to be able to product enough of their own whatevers to pay off the equipment (with a colony credit line, or 'company store' levels of human plotting) - but for the colony to exist, they had to think something was there worth investing in, yah? And they're going to be paying off the shipping cost with stuff they think will be worth it - ten years' (or whatever) cost for shipping, but it would take fifty years without it - so forty years to make up the difference in the price.
