Terminated contract. Is it necessary to bear the cost of returning the routers?

Good day. The internet provider terminated the contract due to a 3-month delay in payment (39 PLN x 3), which has already been paid. The router and TV modem need to be returned to the central warehouse in Mielec. They require 2 medium-sized boxes that weigh over 1kg. I want to avoid shipping costs, as it would amount to around 20 PLN or more for a 19x24x27cm-sized package. The installer delivered them, so maybe the installer can also take them back? I asked, but for now, they only gave me the shipping address and ignored my question about pickup. Is it possible to return them somewhere instead of shipping? The company wants compensation of 2x 300 PLN for not returning them. The contract has been terminated, and I don't want any additional costs. Is it possible for the provider to pick up the modems themselves? Since the installers constantly drive around the city, they could do it while they're at it, considering it's a big city and the building is surely on their way.