Hi again,

It took a long time to test your issue.
– Use default theme
– Deactivate all irrelevant plugins
– Clear all cache on the browser as well as on the website
However the problem is still not resolved

Then I downloaded your website data backup and installed it on my computer – localhost. I check out and the cart is still working properly.

So your problem is with the hosting hosting. Are you using Cloudflare, CDN or hosting cache for this website?
It was the hosting cache that caused this error, please check and clear all the cache on your server to resolve the issue.