It's voluntary, until your customers require it. Thailand's e-Tax Invoice system is becoming the default expectation.

Thailand's Revenue Department operates the e-Tax Invoice & e-Receipt system as a voluntary framework, but adoption is accelerating as larger businesses and government agencies increasingly expect digitally signed tax invoices from their suppliers. Documents need a digital certificate and, depending on the method used, a time-stamp from the Electronic Transactions Development Agency.

Once you're registered, every e-Tax Invoice has to be digitally signed and submitted to the Revenue Department, then retained in a way that stands up to audit. CargoWise Thailand e‑Invoicing Integration handles the signing, submission, and retention from inside the system you already use for shipments and billing.