I've never mailed a sample, but I deal with USPS daily.
You can't trust that the employees will know all the rules. I think you're better off putting it in a box to obscure the contents so they don't get worried. I like the idea of omitting the word 'Labs' as someone earlier suggested. I'd also enclose a printout of the Blackstone letter, just in case it gets opened somewhere in transit. That should hopefully quickly resolve the issue if one occurs.
It is possible to get tracking on First Class Package, but not through USPS' web site. They do sell tracking for it at retail for added cost. I think tracking is only available for first class 'package', which is more expensive than 'flats' or 'letters', but due to the thickness these should always be a 'package' anyway.
If you happen to have a Paypal selling account, you can print first class package postage there and it will come with free tracking. Other online postage services also probably let you do this.
USPS doesn't sell first class on their own site, because they apparently just want to be difficult. But they can do it with tracking at the retail counter, I've done that several times.