Although, I didn't order via Amazon but had a phone privately shipped from a family member, my phone was not on the NBTC approved list, I basically was in a similar situation a few months ago.
It took me a few days of reading and calling around to find out what exactly had happened and what could be done.
Officially, all phones being imported to Thailand require an NTBC license. If the phone has been approved by NBTC and is on their list, in theory, some have done it, one can go through various bureaucratic steps and apply for the license. If you succeed, most likely it will take a decent amount of time, effort and some expense.
If your phone has not been approved, and thus no license can be acquired, the only way to get it, is to pay a well connected broker, who will persuade the customs officer to allow it to be imported without a license. Although, I can't speak for all brokers, but expect it to be 10k upwards.
Possibly, but without certainty, it could be that the fee would drop without an agent in between. But that would require you to have face to face time with the right customs officer.
Then there is the option of having it sent back, but here one enters a catch 22. In order for it to be exported out of Thailand, it requires an NTBC export license, which one either can't get or has difficulty getting.
The final option, which some have succeeded in doing, is to file for a return from Amazon and buy the phone in Thailand (if available).