it looks like Better NPC is denying damage, it depends on NPCSpawn, i checked its code and i can say that:
- it checks for damage range, it can be controlled in config, if damage range is more than in config then it's denying damage
- if npcspawn can't target couriernpc, then it won't allow damage too, probably it's the case?
to check is npcspawn can target other npc, it uses next checks:
- checks for npc attack mode from config
- checks for npc sense range from config
- then checks whitelist and blacklists
i think it can be fixed by editing the config file without editing code
however i only checked npcspawn, maybe betternpc have some other checks but i don't think so