post up
In basketball, to assume an offensive position between the free throw lane ("the paint") and the basket (an area known as the low post) with one's back to the basket and a defending player, so as to receive the ball in a position where one can attempt to shoot or move closer to the basket. If you've got a size advantage, post up and I'll get the ball to you.
post something up
to record a transaction in an account. I'll post this charge up right away, and then you can check out. Please post up the charges a little later.