US Postal Service Priority Mail® Estimated delivery 2-3 days
US Postal Service Parcel Post® Estimated delivery 2-9 days
With the cheaper option, the seller waits a week to mail it -- hence 2-9 days.
Well, OK, maybe I can't prove it. Ever watch a tracked parcel go no where for a week because the seller bought the postage via paypal but hasn't actually taken the parcel to the post office?
"No, you don't understand. I bought the tracking number, paid too much, so bought new postage the next day. You'll get the package in 5 days, honest."