I've developed a workaround for the sales tax issue. Someone in the forums got the same idea. Xero Support proposed a solution as well but I think it's awkward. Support says they've got this issue on their list of suggested improvements.

Pine

On Sun, Dec 13, 2015 at 12:44 AM, Pine W <wiki.pine@gmail.com> wrote:
Hi all,

Awhile back we discussed accounting software on this list. Cascadia Wikimedians decided to use Xero. I just want to post a note for the benefit of other affiliates that for the most part I like Xero and particularly appreciate that their end-user support seems good so far. The main downsides are that their visual design seems to be optimized for mobile and is less user-friendly for desktop, and it's especially mediocre for high-resolution displays. Also, Xero handles sales tax incorrectly for the way that state and local sales taxes work in the US; I believe that Xero is handling sales tax in a way that is intended to be compatible with VAT, which is quite different from how sales tax works in the US. If you're considering using Xero, I hope that this info will be helpful to you.

Regards,
Pine