Thank you for this additional information. It helps me to assess your situation better.
The conclusion is that you were fortunate to have had to pay only 35 Baht.
The details:
1. With the declaration of medicine, customs at many post offices would not have allowed delivery of the package. Perhaps your post office does not have a customs official looking over their shoulders.
2. The declared value of the contents must reflect the cost of purchase of the content, not the amount paid by the recipient (in your case zero)
3. Packages with a value of not more than 1,500 Baht were, and perhaps still are, exempt from import duty. It appears that no import duty was assessed for your package.
4. Quite recently, the Thai government decided to assess the prevailing rate of Value Added Tax (VAT) on every import with a value of 1 Baht or more.
5. The zero declared value was obviously false. You can imagine the burden on the individual post offices and the delivery delays if every package had to be referred to customs, which incidentally would probably incur an additional fee for customs inspection. I suspect that a rule was established and perhaps published in the Royal Gazette to the effect that post offices are allowed to assess and collect the VAT directly for packages exempt from customs duty and, where the declared value is zero or suspiciously low, to assess VAT based on specific parameters, eg weight of the package and/or declared category of the contents.