Support. I honestly did not know that you could sell items to Paul I guess you learn something new everyday. The only other way to go around this would be to alch everything.
I feel that if you have a drop warning value set in your settings, it should also be applied to items you attempt to sell in the shops (and trade post). Support.
Unfortunate situation. Alternatively, i think we should have another tab within Paul's shop that is empty, and you can sell your junk items. On top of this, make the items appear like they would in a General Store on osrs, in which you can instantly buy back any accidentally sold items for the exact same price you sold it for.