It's a good deal, better than the regular services.
I sold a heavy textbook on eBay, wrapped it up in brown paper, and used Canada Post's website to calculate the postage rate. It was about $16. I put actual stamps on it and took it to the post office to mail it. The agent confirmed that I had put on the correct postage, but told me that a better deal would have been to buy the prepaid box for $13.49. Lower postage and a free box, what's not to like.