Thanks everyone for the great response to this.
I went to the post office today and it appears I was very close with my estimates. The included insurance is up to 25€, every additional 100€ (so for 26-125€) costs another 2€ postage. I'll be declaring all shipments correctly, I don't really see the need to undervalue a shipment that is probably around the 15-25€ mark for most anyways, and only the declared value would be refunded on insurance claims.
I've also talked with Kin and we are looking into offering 3 more colors, he'll let me know how close these can be reproduced as the factory uses dye-sublimation which is known for being hard to work with for saturated colors.
I'll now go hunt down a seasoned GB runner on Slack/Discord for how to best setup order collection (a google form probably) and think we can launch this at the end of the week or so.
Edit: finished setting up the google sheet, I'm confident that all information is collected. Waiting for kin to confirm the new colors now.