I recently found pirateship and it is indeed significantly cheaper than retail shipping prices. I told my mom about it and she refused to believe it isn't a scam!
How does pirateship work? Where do the savings come from if they're still using USPS under the hood?
I believe they just get a commercial rate from the USPS and they add a small percentage onto each transaction for themselves to make their money.
They use Cubic pricing which Usps offers to high volume customers.
USPS, UPS, FedEx, etc. all charge consumers "retail" prices that have very high profit margin for them. Businesses negotiate significantly lower rates that are closer to the real shipping cost. Pirateship acts as a broker, allowing people to access those lower negotiated rates.