Hello Ian,
this is a well know issue and unfortunatly because the underlying library J2Store was based on (F0F), is not longer maintained for the latest versions of PHP.
If you need error reporting on, you need to modify your php.ini file to ensure all warnings related to deprecations are removed. You will need to set the error reporting to 'System default'.
In the php.ini file, look for error reporting and add or uncomment:
error_reporting = E_ALL & ~E_DEPRECATED & ~E_STRICT
Deprecation messages are not errors, but are here to tell you that the next major version of PHP (PHP 9) will no longer accomodate the outdated code.
We are building J2Commerce v6 from the ground up to get rid of F0F so that this won't be a concern in the near future.