You could also make a journey to the bkk airport cargo area and talk to the FedEx representatives there…
obviously call first and setup an appointment so you the FedEx representatives and Thai immigration can all sit down and talk
Importing things to Thailand is super expensive so make sure you bring sufficient funds to clear your items out of the customs frustrated items customs hold area
the first time you have to go do it in person or online
*1st time is your inaugural 1st time or any time you re-enter country from an international border via international travel and stay static in country for 90 days (I have to clarify before we go down that rabbit hole)