I would start with:
When Box Picked up > Move Box negatively along the Y axis. Seeing as your players height will never change, this shouldn't be too difficult.
I remember in Mario 2 they used to snap the object to be infront of the player and then move it up and above the head.
Set your animations, like "Arms Up", if you have something like that.
Enable Pinned to Player. Set your "Carrying an Item" variable.
Then have an event where, if the player presses A, enable bullet movement and unpin on the carried object.
Have a sub-event, that will set the direction of the bullet based on the direction the player is facing. Facing Right set the angle of motion to 45 degrees, and facing Left to 135 degrees.
I would also disable collisions on the object once it has been thrown so that it falls off the screen. Then destroy it. You can handle enemy collisions (if you are using the object as a weapon) by checking for overlapping.
Unless you want to keep the objects you are picking up, for like building blocks or something.
Does that help?