When I update my database schema with drush entity-updates. I get an error
Sounds like you are trying to update existing field order_total in uc_order. You can't update it since you have some data in it already. Remove content that uses it (orders), and try again.

Also note that drush entity:updates currently doesn't support all changes (e.g. changing table keys). See this Change Record for information on providing a hook_update(): https://www.drupal.org/node/2554097