I'm thinking the chests could "burn" rocket fuel or something? and maybe steel? or LDS?

I love the concept but I want it to cost something for me to be comfortable using it.

also, it would be cool if instead of a chest (feels too "small") we could reuse the cargo bay graphics or something and use them. If it used the actual pods too that would be super cool.

I was thinking about that but I'm pretty sure the way Space Exploration did their delivery cannons was a recipe, so I'd need to do recipes for everything that could be transferred. I could maybe look into some sort of "delivery logistics supplies" chest and stuff just gets pulled from that on every delivery, but that might be complicating things more than I want for this mod.

I feel like that would kill half the point of using the mod as I could just...launch a rocket from the planet if I'm spending a bunch of resources. Throwing something off the side of one platform and catching it with another really doesn't cost much if you think about it.
Though one could argue you could do a "barrel" like concept where the chest needs to use barrels with its launches, and they're reclaimable on the other end, so that there is some logistical planning that needs to be done around containers rather than yeeting 100 copper plates into space and having them all stick together by themselves.

That said, base game, it doesn't cost materials when sending materials back planetside, so if they don't even need to contend with the atmosphere when launching to another platform, should it conceptually cost materials in any way? I already drop a hundred steel per minute onto my planet from orbit "for free" why would it cost material to instead just toss it to another platform?

If I do add anything for launch costs I'll make sure to add a setting to enable/disable the costs. It'd probably be fairly cheap anyway, more of a slight logistics puzzle than anything else.

Some power or perhaps fuel cost might be interesting - the cost of the chest could include the "delivery drone", but you'd have to put fuel/oxidizer into the chest to refuel it between deliveries (similar to how you'd recharge bots between flights).

For balancing, it might also be interesting to force these chests to be adjacent to the platform hub/cargo bay (or maybe allow 1 block of separation, to allow inserters inbetween?) and scale them up to achieve similar storage densities as cargo bays (20 slots per 16 tiles at normal quality - so maybe 5 slots for 2x2 tiles, or also 20 for 4x4?)...

I think some cost would be interesting balance change. Unlike mentioned, launching another rocket from the planet is not the same as adding a cost to this. Sending stuff down only to launch rockets again needs the infrastructure on the planet, makes a delay in delivery, is pretty idiotic in the first place and the cost is much higher than what the costs for this should be.

Items already have a launch cost integrated via the rocket launch size. Maybe you could use that value to make the cost dynamic without inventing a new scaling system. Use the "rocket stack size" and instead of a full rocket the cost is maybe a steel barrel and some fuel each delivery. A steel barrel and some fuel makes it feel like its a makeshift mini rocket or launch pod that is sent instead of just teleporting stuff to the other platform.