Lost package with An Post following incorrect postage

[KJPF]

[FONT=arial, helvetica, sans-serif]A couple of weeks back I paid a visit to a post office with two parcels addressed to the UK. These packages contained goods which were being returned to a clothing distributor for a combined refund to the value of £860 pound sterling.
[FONT=arial, helvetica, sans-serif]As per the distributors instructions I requested that both packages be sent via registered post so the whereabouts could be tracked online.
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[FONT=arial, helvetica, sans-serif]A week later I received word that one of the packages had arrived however the second package had failed to appear. When I checked the postage receipt to investigate the item's location via tracking number, I discovered that both packages were sent via standard post and not registered post as I had requested.[/FONT]
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[FONT=arial, helvetica, sans-serif]As of time of writing the package (with a value of £660) has still not turned up at it's intended address.

I've emailed both UK and Irish customs and both have said that the package is not held up. The distributer have checked their warehouse multiple time and I've been told it's definitely not there.

[/FONT][FONT=arial, helvetica, sans-serif]I have been in touch with An Post and they have opened an investigation but I'm not holding much hope.

Is there anything more I can do? Are An Post liable considering I specifically requested registered post and they sent it standard?
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[BillK]

Did you pay for registered post and can you prove it if you did?

 

[KJPF]

Unfortunately not. When I checked the receipts they were marked as standard. Postage wasn't cheap (€22) so I assumed it was sent registered, I never bothered checking until the time came to track the parcel.