Hello,  I see you are new to the forum,  welcome. First is it nice to have participants in the forum.  But I do suggest that postings should aspire to be correct. 

With the said,  let me correct your posting: The DEFAULT USPS  is NOT a third-party.  It is a core dev application.  As you can see that in the posting from Basara (who is a core developer).   

There is a third-party enhanced USPS integration (WHY2 is the developer of that extension)   And  it was updated  in July for version 1.3.3  with the changes in the services executed by USPS.  You can verify this at our manual here  https://why2central.net/docs/usps-changelog/

That manual can be a good resource for how to even with the core USPS.  We just added some features our clients needed.

I hope this add clarity for forum readers.

Cheers
Lee (the WHY2 support team)

You have pretty well understood it.  Be sure your two sites are NOT on the same machine and have different IP addresses.
the URL is the SAME for both sights -the difference is in the DNS (nameserver) which is controlled within your domain nameservers info at your registrar.

And you have picked up on the syncing issue.   As to what "big companies" do,  there are several solutions.  from simple to complex.

What kind of volume to you have? That will help you decide how much effort and cost you need to insure your info is relatively in sync.
Various solutions -  you can move your DB to one different location and let both app locations write to the same DB -  then make more frequent backups of the DB  in a location other than your site location.   eg.  if you have Softaculous you can set your backup for just the DB  and save the backup on a different location like a dropbox account or AWS if you use that.

Look into what your backup options are from your hosting company -  are you having a daily backup with several generations auto ran  and stored off your site server??   
Your hosting company should able to provide you more specific options that work within their framework

Do you get notification when your site went offline?

There is also the ability to have a hot-swap raid backup  on some hosts -  that does cost more,  but what it is two drives assigned to your account -  and they both get written to almost simultaneously,  if the main one goes down,  the hot swap is actually pulled physically and placed in a new location -  most host who provide this monitor your site and if there is a mechanically failure of a drive they do the swap  quickly.

Hi CoreDevs,

So it is now 4 days after this announcement -  can you please get you Demo updated on AbanteCart.com  ,  it is still 1.3.3 as of a few minutes ago.

AND please update Softaculous ASAP.

We are all busy,  but these items should be done when you release new versions.

thanks

papafrank,  I believe you are probably not in the correct place with UPS.   You do not use your regular UPS shipping account,

You need to get an API key from UPS. Go to the Developer Kit

https://www.ups.com/upsdeveloperkit?loc=en_US/

Once you have the developer account,  request an access key.  That is a  key for using the XML rate lookups.

Update-  WHY2 investigated ,  and found the issue for Ray.   We report here the findings and the fix as as it might help others.

In the Database - settings table config_currency was still using CDN.  I seems when Ray changed the currency code to CAD  the config_currency did not update (cause is unknown) 

The fix was to change in Admin -  System > Settings > Store Details   the field Base Currency ,  we changed from Canada Dollar to Euro,  save,   then changed it back to Canada Dollar.    this time the config_currency in setting table  was updated.


The WHY2 Support Team

https://www.paypal.com/us/webapps/mpp/country-worldwide

this is the countries  PayPal supports -  Note  that the statement -  We are available in more than 200 countries/regions and support 25 currencies.

Seems the issue might be the currencies? 

We're the extension providers for Canada Post -  and it requires you to use CA to get the correct lookup rate.

Try CAD for your Canadian currency

The support Team at WHY2

Check your system to see if the requirements are met
https://abantecart.atlassian.net/wiki/spaces/AD/pages/3768390/AbanteCart+System+Requirements

Seeing any errors in your admin error log?
I see there are several extensions included in the Mercato -  perhaps try turning one off at time  until you find the problem?
Then I would try the developer again.