And no legitimate company would buy something like this from anyone with a resume like Lazar's, not to mention from someone who is offering to deliver it via a patently illegal method (USPS flatly prohibits mailing radioactive materials).
According to the article, the USPS doesn't flatly prohibit mailing radioactive materials. It allows people to mail trace amounts of radioactive materials by ground as long as the package is properly labeled and packaged, and other hoops are jumped through.
This website explains the USPS' requirements.
Also from the article:
A woman at Lazar's company, who identified herself only as "Michelle," said the firm sells polonium-210 in "small, small, minuscule" amounts ... What we carry is so small you can't see it with your naked eye." She said she is only an employee at the firm and doesn't know where Lazar obtains the polonium-210.
Riiight.
Not clear that this particular substance could be mailed, but from the USPS website it appears that the flat prohibition applies only to mail via a class that can travel by air. Small amounts of some (but not all) radioactive materials can be mailed by surface mail with strict packaging requirements and activity limits.