According to Censor.NET citing TSN, people walk through the crossing point and come back with vests for soldiers serving in the ATO area. The activists usually do it late in the evening. Volunteers say that one vest is one life saved at the front.
People mostly buy flak jackets at warehouses for 150-200 Euros, transfer them to Poland, and from there bring them through to Ukraine on themselves allegedly for personal use. There is no other legal way to bring military equipment across the border.