If you actually book the shipment through any of theses postal agencies, the rate will generally be lower that going direct. They buy up extra capacity from the carriers at lower rates and sell it on. But like gas prices, they fluctuate daily.
Here's your parcel via Postage Supermarket
this morning. Downside is they generally prefer a business address at each end if possible.
All the best,
Pause, take a breath and ask yourself: "What would Rastamouse