If you bought a random number of £x.99 items, on average you'd end up putting roughly the same total amount in the box. Of course that wouldn't help if you always bought three, but to counter that there would be somebody else who always bought two and ended up putting more in the box.
Yes you can quibble with the details (and I'm sure somebody will), but the point is that on average it all evens itself out, more or less.
Speaking for myself, I put everything below 20p in the box. It's only at
20p that the ratio of value to size and weight becomes acceptable.