In endgame Steve goes back in time to spend his life with Peggy Carter. But he knew that during a certain point in time Natasha, the black widow, was in Budapest. Steve could have very easily picked up some Pym Particles and taking them to Budapest to give to Natasha along with the time travel device.
This would explain why he had to wait (and age) in the normal mcu timeline to get to the point where he was waiting for the winter soldier and falcon and hand over his shield. he did not have the time travel device with him as he had handed it to Natasha in Budapest
it would also explain how the Tasha apparently has a handful of pym particles in one of the trailers. It is the most simple and most effective way of explaining her possession of them without adding more time travel complications to the mcu.