I had the same issue, i had a car part sent from Japan and received the same message. After contacting Fedex, i explained that i am not an importer and it is just a small car part for private use, i wash told i had 2 options, One to use an independent customs clearance company or get Fedex do the required import process at an additional cost, i had to send ,my passport copy and a signed form that Fedex sent to me to. in addition to the passport copy i needed to send the invoice or proof of purchase price, which i sent to them.
I them got another email with the total cost including the import tax, in total it cost me another 1200 tbh to receive the package with my part. and today almost a week to receive the item once it arrived into Thailand
Just seem to be a money grab by Fedex