D3 is the pure Dunes coins offer - you get 5 pulls for $5 (USD) and some battle items as goodies.
Beyond that, there isn’t a pure Dunes Coins offer yet. There are a few $100 offers in the past that offer Dunes coins plus some goodies (all subpar offers):
In addition, T3 of FToI is a good place to get Dunes coin because it is a plus value offer ($1 per pull plus gems), and you can stock up on challenge coins since the Challenge event will be coming up soon. With FToI, you must pay a gems tax and an unlock tax, which is unfortunate.