If arriving in Bangkok look for a SHA+Extra hotel package. They include transport and the PCR test. The test is done at the hotel, either on arrival (or at a test centre en-route) or within a few hours.
If arriving in Phuket the test and transport are not part of a hotel package (but many will arrange transport for you). The test is done at the airport.
For Phuket airport tests, book and pay here.
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