Problems with paying FedEx duty & tax invoice online

Hi all, 

I am wondering if anyone has had difficulties making FedEx duty & tax invoice payments online? 

Basically, back in early August, I got an email from FedEx asking me to make an online payment for import charges for an order I made from the US. The amount was €26.06. 

I had no problem making this payment, so I paid it via the link provided, and thought that that was that. 

Then last week, I received another duty & tax invoice from FedEx in the post. This time, the amount they were asking for was €13.77 for a disbursement fee and original VAT. I assumed that this was some additional fee that needed to be paid but they're only asking me to pay it now. 

Again, I am willing to make this payment, and I went to the link they provided in the letter: https://www.fedex.com/payment/

But when I entered the invoice date and invoice number (from the top-right hand corner of letter I received in the mail) in the link above, I get the following message (I have X'd out sensitive info): 

"Invoice XXXXXXXXX from XXXX-XX-XX has an outstanding balance of 0.00 and requires no additional payment." 

I sent FedEx an email about this to "ieinvoicequery@fedex.com". I got a standard automated response saying that a ticket had been open, etc.. But this was a week ago, and I haven't received a response. 

In the meantime, I did some searching online and found another link: https://dutyandtax.fedex.com/ie/en#/

I entered the invoice number and the amount, via the screen above, clicked "Pay" but I'm presented with an error saying: "We are unable to complete your transaction".

I sent another email to "ieinvoicequery@fedex.com" about the above, and also referencing my previous issue. No response yet. 

I have done some reading online and I found out that people in a similar situation to my own are receiving letters from debt collectors contracted by FedEx even though they're willing to make the invoice payment. I am terrified that this is what's going to happen to me, as it will have damaging effects on my credit rating (I am currently in the process of applying for a mortgage). I want to make this payment to FedEx, but their online facilities aren't working and their customer service are ignoring my emails. 

Has anyone else experienced these issues, and how did you resolve them? 

Thanks!