Oh please, for the sake of eco-friendly post delivery everywhere, do try and get a good saddle and seatpost to switch to! You'll find plenty of good advice on picking a saddle, here on TE. For starters you can check out which saddles we use, many of us have them posted in our signature. Make sure that the seatpost you get is the right diameter to fit the bikes you use, there are a few different sizes.

I wish you could get good bikes too, no-one expects post delivery people driving cars to drive ancient beat-up cars, do they?

Wishing you lots of luck!