Call them or visit their office, either of those approaches demonstrate more credibility and less chance of time-wasting, non-serious info gathering than an email. They specifically offer a Bangkok Bank account opening service on their website and the fee is B5,000. VAT is 7%, not 17%...if they didn't respond to you how did you get that quote?
Opening of Bank Account Assistance
Siam Legal International provides a hassle-free service for foreigners without work permit to obtain a Thai bank account in Bangkok as follows:
A completed form by Bangkok Bank
A full reference letter for short stay or long-stay foreigners, with an official stamp from Siam Legal International
A copy of ID card of the reference person, with an official stamp from Siam Legal International
Applicant's original passport
The applicant may retain Siam Legal online. After payment, our staff will schedule an appointment to visit our office and the request of the application documents from Siam Legal International. We only require your passport and the process takes only less than 30 minutes and the applicant can proceed to Bangkok Bank, Exchange Tower branch on the same day.
The assistance for opening a personal bank account costs 5,000 THB.
https://www.siam-legal.com/other-services/opening-a-bank-account-in-bangkok.php