I use GnuCash for two companies and personal accounting. Love it.

I use Calibre and its web server locally. Works great.

I do the same. Using VPN, proxy, Firefox container, tracker blocking, the whole 9 yards. I haven’t seen price change based on location/purchase history. I’ve been looking for signs of this for a while. Prices do fluctuate quite a bit though. The site might have auto-selected a different “ships from” option for you, depending on how you’re logged in.

I followed up with customer support. They said go ahead with the order and they will refund me the amount after. They gave me a $5 credit for my trouble. They said they would escalate this issue to the development team.