This all sounds very informal. I would bet there never was a written agreement that she pay you anything, let alone that you would be allowed to keep her belongings until she paid. In other words, you're just holding her stuff hostage because you think she owes you money. Don't store or ship anything. Tell them to come get their things (at their expense) within 30 days or you're throwing them away.
You need an attorney. You are on dangerous ground. Your actions could be seen as theft. You can not take possession of another persons property on your own because they owe you money.
This is the sort of problem that can arise when there is no agreement in place to cover all of these contingencies. You may very well need a lawyer, but that depends on the outcome you want. First, you have no duty to ship, so don't do it. Second, storage charges are probably not a good idea because there's not agreed storage value for the stuff being in your home. Third, the idea to have them pick up the stuff at their cost is a good idea. That leaves the $1000 you paid to move the stuff in, and you could possibly take that to small claims court. But: are these people the type that you would likely collect from? And in court you would be trying to prove that the ex-roommate had agreed to reimburse the shipping costs; did you have any such understanding? If that's not clear, maybe your best result is to let them take the stuff and be rid of these people. No lawyer needed for that. Good luck.