just use your sons address, remain in the UK taxation society and keep your bank account, they need not know it does not matter, you don't have to have an international account. 👍
Books are very complicated actually, strict rules on the content and if educational. I once had a client want to send a library to Afica to a school, guess what, they wouldn't allow it. Other items had to have h&s certificates otherwise they couldn't go and electrical items tested and certified, it goes on and on, it becomes too much hassle.
Understand your best intentions, do alot of research first. 👍
I worked in freight services for 35 years, you can find a charity first to donate all to, then they bring in as charitable items, they will know the rules. If you want to bring in as personal effects then you need to catalogue the items and value for customs (not insurance)
Then find a specialised personal effects carrier. They will check and confirm everything with their agent in Thailand prior to shipping.
Costs have gone up immensely so I would image its cost prohibited and you would be better off selling in the US and make a donation instead.
Be humble and point it out, apologise and say it was a miscalculation, if they do anything it will be the fine, like others have said its an easy mistake to make.
Normally a visa agreement is in place so I would have assumed 30 days tourist is no issue, chinese come in the 100's, so does everyone else for that matter 😉