It would have been helpful if you added a screenshot of your setting, or copy pasted the text.

https://arastta.org/docs/user-manual/system/settings/server
«Display Errors: Whether to display the PHP errors, warnings and notices produced by your store. This should be set to "No" for a live store.»

If they still display after set to "No", clear cache. And if it still displays, get your host helping disabling the "override".

I have the settings the same.

The fedex one is the error about the missing postal code.

The canada post error is a coding error made because the postal code is empty.

They are kicked back most likely from the extensions themselves and turning of the error messages doesn't seem to include those by the extensions.
No biggy.

Yeah, the setting is for PHP errors and notices only. Not important system messages and info.

For the postcode message etc. I think there is enough post already about why and how to fix.

I don't know enough to fix the Canada Post one myself and Andrew Tite said he would take a look at it. Hopefully he comes up with a fix.
I just did an override of the Title and instructed users to add a postal code to remove the error. Cheap work around.

OK,

I have considered a change to make it possible to get correct sorting for postal code and city, based on choosen country. Didn't get any respons on making this improvement to the core, but maybe I'll make a modification for it. Thought about adding contry and zip to the first step, and that would solve your issues too. I'll let you know if I do it.

Interesting idea. Have the ability to set a default postal code for all new registrations.

I tested it using Santa's postal code H0H 0H0 on the Canada Post extension.

The coding error goes away and the error appears: Error 1740: The Postal Code is invalid.

That's a little more acceptable as an error.