All of my greeting cards are now sent ‘naked’, without a cello sleeve.
Unframed prints do need some protection in the post. I'm currently using bags made from PLA, a corn-based plastic which is biodegradable, compostable and recyclable under the right conditions. PLA has a lower carbon footprint compared to cello but it's not a perfect solution as it can still take a long time to degrade in landfill. I'm currently looking at better options.