Require proof documents for the charges

I bought a dress on the internet from Beauty dress I then received email from TNT stating that I must send them invoice and proof of payment for my purchases of which I did. After many to and fro mails I asked them to send me the invoice for the charges. They told me customs detained my parcel and there is a penalty charge and lots of other charges. All I am asking is to send me documents from customs requesting these chargers. I also find it strange that they are dealing with me and not customs.