Demand for duty and VAT

emmyroo_2

I ordered from the EU back in April, the cost was £134.81 of which £22.49 was VAT.
The items were delivered with no hold or delay or charge at customs.
I returned one of the items valued at £44
I now receive a demand from DHL for £57.88 for import VAT and fees.

I was below the £135 threshold
The items were delivered with no charge for release
Why am I now getting a demand 3 months later threatening debt collection?

Where do I stand here?

Thanks