Nobody can tell you anything without seeing your footage, but the truth probably is that you already shot it the wrong way and it can't be used, anyway.
To remove a person holding a bag from a shot so the bag looks like it is floating you need two shots that are identical. Put the camera on a tripod and shoot a "clean plate" that does not have your person or the bag in the shot. Now shoot the main action with the person and the bag in the shot. You then layer the shots in After Effects. The Clean Plate goes on the bottom layer and the main action goes on the top. You then use rotoscoping to create an animated mask around the bag so only the bag shows. Because you have a locked down camera and a clean plate this mask only needs to be accurate around the hand holding the bag as long as you keep the person to the side of the bag. If the person holds the bag in such a way that their body is behind the bag at any time in the shot then it's even harder to do the masking. If it were me I would get a stick, tie some black thread on the stick that was strong enough to hold the bag, then tie the string to the bag. This would make the roto even easier.
If you want to throw in a hand held or moving camera then you must do everything that you can to insure that the shots are as identical as they can be and you need to learn about tracking, stabilizing (not Warp Stabilizing) and time remapping to get the shots lined up as well as the timing.
This is really basic stuff. To learn the techniques needed type "rotoscoping" or "masking: in the search help field at the top right corner of AE and read up. The search help field is the most useful part of AE for new users and only about 2% take advantage of it.
Thank you so much for your advice! I will make sure to use your steps carefully!