Upgrade. What a shambles.

TBH, I was about to give up with OC after trying to upgrade from 1.5 to 3..0.3.1 because there are soooooo many issues with this upgrade.
I consider myself to be EXPERT level at website builds and currently code in PHP and design DB's and I have never had so many issues with some software, even an upgrade, as I have had with this upgrade. And I cannot believe this must be down to 'user error' or the likes.

I followed the steps in the upgrade text file to the letter but still had numerous problems.
First off as the upgrade itself. I was using the mysql connection to my database but I then read I had to change this to mysqli. Nothing in the upgrade text about that.
Once upgraded, I seem to have lost all my payment gateways and had to go through and set them all up again. Then found that my Worldpay Online Payments extension no longer works and I need to purchase a new Worldpay Business Gateway extension.
Then getting numerous other errors which all seem to be related to the shopping cart. I had setup 4 payment methods but continued to get the error "No payment methods selected" in my checkout. What I had found was that I had setup the Total field in my extensions to be 0.01, which would normally work. But the problem is that my cart shows a total of 0.00, even with products in it! So my shopping cart button at the top of my online shop shows say 4 products with a total value of 0.00. So that's why the payment methods are not showing as they are setup to only activate if the cart is over 0.01! What is that all about??

This whole experience has even tempted me to look at another open source shopping cart as this upgrade, although whoever wrote the instructions wants to make it look as easy as pie, is fraught with issues and I'm afraid the upgrade has almost completely broken my site.

I still have the issue of a zero value in my shopping cart. I have searched for a solution to this to no avail. Some suggested changes to the code - REALLY! Where was that in the upgrade instructions??
Unbelievable!

Almost getting there now, though still seem to have the issue of zero value in the shopping cart despite products appearing in it.

In my Worldpay extension, the Relay Response URL has a weird URL in it which I cannot amend.
It has HTTPS_CATALOGindex.php?route=extension/payment/worldpaybiz/callback in the input box.
There is also an error at the top of the screen stating: "Notice: Use of undefined constant HTTPS_CATALOG - assumed 'HTTPS_CATALOG' in /home/sites/backcaretrading.org.uk/public_html/admin/controller/extension/payment/worldpaybiz.php on line 100"

I've searched the Upgrade notes but nothing!! ;-)