Radioactive stuff, most probably, as it was (supposedly, but Pym didn't seem to refute the possibility) glowing and gave the workers handling it some kind of illness.
It can be noticed that back in Tales to Astonish #35 (1962), one of the first Ant-Man comics, Pym was tasked to make a gas to render people immune to radioactivity, which probably implies studying of radioactive material first. Possibly Endgame was making a reference to that.
Pym has been established in the MCU to be a bit of a "mad genius" at times, tampering with iffy physics (failed Pym particles reduce their targets to goo, remember a certain sheep?). Thus, a (more boring) out-of-universe explanation is most likely to have a funny scene, where Pym and the viewers gradually realise what the package is, as the "clueless" person at the other end of the line still doesn't understand.