if u invoice across a border does not mean its vat free. If you make a service to another then you apply VAT, you do not have to be registered as a company. Here is the definition of tax residency from RD.
DTV holder are tax resident in Thailand if they spend 180 days or more in Thailand. I was freelance, not in Thailand and I always added VAT to my invoices when I was resident in Uk and invoicing my German clients. I have a further issue with this, and this is: If they are predominantly invoicing one client then they have established a "regional office" and must get a work permit and must employ 4 Thai's.