Hello on the GUI plugin page it says:

Note: Economics Design flaw

• There is a design flaw with economics that does not allow/provide other developers a way to know when a player has reached its economics configured balance limits for both earning currency & losing currency.
• Until Economics receives an update for "API" methods/hooks to allow other plugins to know this information.
  It is recommended to not enable or use any of economics built-in limit functions since the shop/any other plugin will not be able to know when a player has hit a limit and will continue trying to take/give them currency regardless.

The issue as I pointed out to the GUI Shop maintainer way back, is they are assuming transactions are successful rather than checking the return value of hooks from this and other plugins it depends on. I offered to add methods to satisfy them despite that, but ideally the plugin should be checking returns instead of assuming as the assumption is what is causing the issue.