1. As of now Hotcakes does not provide functionality of applying promotion codes for user supplied price. We have user supplied price and we also want promo code. We have done some work around that applies promotion code but in that another issue occurs that when application pool recycle sub total value set to zero and when we revert our workaround subtotal does not change to zero value. So our workaround is not a perfect solution for our requirement.
2. Also, When we enabled promo code for both offer price and shipping, Offer price promo code not working(It is working fine when no shipping promo code enabled).
Could you please check and provide your feedback and let us know if you need more details.