For some reason today when shipping items to Poland, I can only set the customs section to DDU or IOSS DDU.

This means that I have collected the IOSS VAT (correctly) but the customer will still have to pay the customs duty. Is it just Poland for some reason - yesterday I could select DDP (duty paid) for EU countries but not sent anything to Poland since this came in.

In any case, why are Royal Mail then charging a £1 handling fee on top - what exactly are they handling?

It is a nightmare as I have collected extra from the customer to cover the flat EU customs fee (€3 plus the £1 handling fee). Now I feel as though I need to repay the €3 back to him.

Presumably the Royal Mail £1 handling fee will not cover the Polish postal system handling fee for collecting the customs.....