Billing - Auto Pay Top Should Allow Negative Numbers

Currently clients only have the option to set up Auto Pay when they reach a positive number over $10. However, ShipEdge allows us both a minimum balance model, and a maximum I owe you model. If both models are to be truly useable then a client should be able to set a trigger that if they receive a certain amount oxed they can add $xxx.xx amount. Example, if I allow my client to owe up to $500.00 before going into low balance status, can we have an that when they reach $200.00 owed or a like number that their account is the ncharged to top up?