I hate that corrugated plastic. You NEED to put in chopsticks or something on the sides so it doesn’t crush, and they’re just a big pita to take seeds out of without mixing or damaging any compared to other shipping containers.
Centrifuge vials are cheap on amazon or ebay. Can usually get at least a couple hundred vials for less than $10. The 0.5ml should do ~15-20 seeds. The 1ml vials will easily fit ~30-40. May or may not be compatible with rollers in an envelope.
If the vials are too small to deal with due to arthritis or other issues, Lacon Sample Containers are a bit more expensive but sooo much easier to work with. These are what i’ve seen Sebring use for a lot of the seeds he sends out. These should also easily fit in an envelope for the rollers.