We suggest ordinary national postal service and a bubbled envelope. Private couriers or tracked packages are more problematic in this sense. In any case, problems with small envelopes using national postal service are very uncommon. Around 99% ofn samples are received and, in case your sample is lost or seized in customs you can send it again without additional cost for testing service
I was wondering this as well, how to send out a bar send out a sample that is a tiny bit bulky like a xanax bar or MDMA pill without having to go into a post office / be on camera printing postage, can you just buy $5 worth of stamps and stick them on a padded envelope?
You can buy stamps and put it on an envelope with no problem. The usps sells single stamps that are like $6 or something. The only thing to consider is that if they package is above 13 oz (obviously not for sending to EC, but general info) and has a stamp(s) on it then that is considered anonymous mail and will be sent back to the return address. Packages over 13 ounces must be mailed over the counter with a clerk at the post office.
Otherwise, no one cares if you mail something with a lot of stamps on it.