"but the package is actually shipped several days later.:"
 
How could you possibly know when the seller actually shipped?  If the seller prints a postage label on Friday afternoon, drops the parcel in a mailbox that same late afternoon or the next morning, the postal system will not pick it up until Monday (or Tuesday if Monday is a Holiday).
 
Then we expect the postal service to efficiently enter a tracking into their system every time.  We all wish it worked that way but we know better.  The postal system is not always efficient.
 
And yes, there will be instances when sellers go to the post office once or twice a week.  Often they will mention it in their listings. So a postage label printed on Tuesday afternoon may be delivered to the post office of mailbox on Friday.
 
Can you give a specific example (listing number) where the seller was shipping "several days later"?