FYI, the page you supply a link to with the list that says "what doesn't convert" is not thorough enough to be helpful. I've tried to reference this several times hopeful to find info to answer questions from my clients before I convert them and the answers are never there. In particular related to this post, it does not address the sales tax rates. Has QBO improved this so that the sales tax rates will pull in yet?
I can see how this feature would be helpful for you and your client. Currently, importing the sale's tax rate into QuickBooks Online is unavailable. Rest assured, I'll take note of your suggestion and submit this to our product engineers for consideration. Your feedback will help to stay in tune with what our customers need, want, and expect.
Also, you can use the automated sales tax feature. By using this feature, QuickBooks can automatically do the sales tax calculations for you on your invoices and receipts for easy and accurate filings. This can be a reminder so you'll know when your payment is due, so you can file your taxes on time and avoid extra fees.
To know more about automated sales tax, please click these articles: