Using a courier to Thailand

I have to do an annual tax return for a Thai company and have been using the same accountant for 12 years or so. Accountant sends me the documents, I sign and stamp and send back. This year I sent UPS. Clearly marked documents and zero value. Sent from the UK.

First of all UPS said they couldn't deliver as the address was wrong. I notified them that the address was correct but I did advise them that the phone number I had used was not correct as he had changed his number. I got the correct number and advised UPS. Another week gone by so called the accountant and documents have not been delivered.

Phone up UPS who now give me a different story, the documents are in Thai Customs and they want an export invoice. I pointed out these were zero value documents that had originated in Thailand.

Are Thai Customs really that awkward or is it UPS not up to the job? Do I really need and export invoice for valueless documents? What next a Customs declaration on a birthday card?