I usually toss junk mail and don't bother retaliating, but about 20 years ago I got a notice in my Rural Delivery mailbox that I had a postage-due letter. That meant making a trip to town to go to the post office, and paying for the insufficient postage (before the jerk at the counter would even let me see that the "letter" was junk mail). The letter was a solicitation from a company selling fasteners (bolts, screws, etc), and they had sent some samples in the envelope, without sufficient postage to cover the weight of the samples. I was more than a little ticked off at that situation. Luckily, they had provided a postage-paid return envelope for the order form. I went to my shop, where there was a partial roll of old lead flashing (for roofing work around chimneys), and I got out my metal shears and cut up a big fat bundle of lead that "just" fit in the envelope, and I reinforced the envelope's edges with tape to make sure that the entire parcel arrived in good shape. I used the back of the order form (why waste a usable piece of paper?) to explain to them just how pleased I was to be able to cost them a bunch of money. Cost?
Probably a couple of bucks worth of left-over lead flashing. They paid a lot more for the postage. I still grin about that one, from time to
time.