Being charged UK to Eire customs despite order being under the threshold?

Has anyone encountered similar?

I ordered 51 pounds worth of product from Lush UK to be shipped to Ireland.

Customs should only take affect for orders over 150 euro but DPD messaged me today to claim I owed them 18 euro for customs, despite this being incorrect.

I have contacted Lush but am not optimistic I will hear back from them in the next 2-4 days and worry my order will be returned/lost by DPD over this delay.

Should I be contacting DPD to challenge them on this?