Need Advice for Interparcel & FedEx Invoice Dispute

Mayamon

I ordered a parcel service from UK to Singapore on Jan 2023 using Fedex through Interparcel. Paid to Interparcel via Paypal. 
Parcel delivered safely 10 days later, which was perfect. 
Suddenly, last month I received a notification letter to my house saying that I owed FedEx Freight Charges - which was strange as I have paid it through Interparcel. 
Contacted Interparcel & was told that it was just an error and I shall ignore any future email/ contact from FedEx. 
2 weeks ago, I received another warning letter from FedEx saying taht they will take a legal action towards me as the Bill to Party (interparcel) refused to pay!
Now, I am chasing Interparcel to make their payment to FedEx and suddenly they said it was a Duty & Tax Invoice instead of Freight Charges. 
However, on the invoice clearly mentioning a Fuel Charges & Freight Charges instead of Duty & Tax charges (nothing about tax at all). 
When I asked Interparcel to provide me 'Duty & Tax Invoice' copy which they should've sent it to my recipient half a year ago, they stop responding. 

I can't stop thinking that they are trying to pressure me paying something that I have already paid and changing the reason each time I contact them. 

I have also sent a complaint letter to their office last week, what should i do next if Interparcel still ignoring me? I am not familiar with UK Law, obviously I do not want to get chased by FedEx but the amount they are requesting is too much, its £502 

Thank you for any help / comment.