it's not a law but it's obviously a rule of thumb used by immigration. That it's on multiple embassy websites as well as the Ministry of Foreign Affairs website, does suggest it's more than just something each individual IO thought up on the spot, it does seem to be quasi-official internal rule of thumb that came from above. People saying it doesn't exist are right insofar as it's not a law though.
this is the most common Thai socket, it will take any of type A (North American 2 pin), B (NA 3 pin), C (Euro 2 pin) or E/F (Euro 3 pin- will only be grounded if you add a grounding pin in the hole). It generally works best in practice though with US over euro plugs, they stay in better.
Sometimes you get 2 pin only sockets, but I can't imagine you ever had a socket you couldn't plug a regular North American 2 pin plug into. And you will generally have the third pin on most sockets, so they'll take 3 pin NA plugs.
I live here, sockets that take US plugs are the most common. Most sockets are designed to take both US and Euro plugs, which is why they may look different from what you have in the US, but they do actually take US plugs and indeed the US plugs stay in better than the Euro ones. I have a mix of US, euro and UK/HK/Singapore plugs but MOST of my stuff is US plug as that's the plug that works best here.
Amazon and eBay can estimate and pre-pay the tax. It's not a shipping fee then, it's tax. Amazon actually refunds you if the actual tax is less than they estimated. I have prepaid the tax with Amazon and they did refund. Shipping can be expensive from the US but that's a separate thing to tax.
Many items do attract duty and it can bex10, 30, 60%, and you have the VAT on top of that. But laptops specifically are duty free. There isn't one rate for everything. If you got charged only 20-30% on clothes for example that would be an undercharge, as they are 30% plus 7% VAT = 39.1% (plus you pay this on the shipping too).
I have imported a laptop and paid only 7% VAT, no duty, which is correct, as laptops are duty free. There will be a small handling fee from the courier, FedEx and DHL both charge 200B. Thai Post is 20B.
Many other items do have much larger duty, which is what people are confusing. Duty is item dependent, and it's zero on laptops.
it's 7% VAT only, no duty. I have imported a laptop and paid this. A company's shipping fees are their own business, they are not tax to the Thai government. You do pay VAT on shipping fees, but that's the same anywhere.