Thieving/Pickpocketing is actually really really broken,
Crouch/Hide, then cast any buffs (invisibility and guidance is what you want) on Astarion (if you're not playing a rogue yourself) and enter turn based, then just head to a merchant and steal his gold and preferably all his arrows/spellscrolls/potions.
With Astarion + guidance spellscrolls and arrows have a DC of 1 or 2 I believe, gold is a bit trickier because the DC of gold increases if an NPC is carrying more of it.
When you've cleaned the NPC out, just go to camp, long rest and the NPC's inventory will be restocked.
A bit time consuming and totally broken, but if you need the cash for whatever then this is the way.
