The point of this topic was to find out how I pay it and who I call seeing as I don't know where letter is now..
But then people started to say it's scam so now I don't know what to do..
Post 18. The one where you quote the response you received back from the vendor, Horizon Hobby. The one where they say that the package is being held by the postal service and is awaiting payment of charges, (whether they are valid or not in you opinion).
They also say the package has not been returned to them which is what will happen if there is a charge and you don't pay. It is perfectly possible that if the item ordered + shipping comes to >£18 then you will have to pay (unless what you have ordered is zero rated for duty and vat)
Whether or not the original letter mentioned was a scam doesn't actually matter as the way to deal with it in your current situation doesn't involve sending money to Nigeria or whatever.
I've have indicated twice in this thread that (assuming of course you are in the UK as you are not displaying a location) that there is a good chance that something sent from the US via USPS will likely be delivered in the UK by either Royal Mail or Parcel Force. Contact their local depot (PF) and delivery office (RM) covering your postcode. Royal Mail have a number on their site you can call to find out where that is. Parcel Force allow you to search via postcode on their site.
Then phone them up and ask if there is a package for your name and address awaiting payment of customs charges to be delivered. If there is then they will be able to tell you how to pay. You are paying Royal Mail or Parcel Force directly not some dodgy scammer via Western Union.
Worse case is that the original letter was a scam and they don't have your package there with a charge ... you don't pay ... you continue to wait for your package. Best case they do have the package there waiting to be delivered on payment of the charge ... you pay ... get your package.
Alternatively ignore it all ... if there is an unpaid charge then eventually your package should be returned to the Vendor who should ask you what you want done.