Hello Cindy - I managed to upgrade a v1.5.5f site with OPC 2.2.0 and it is functioning so I must have followed the instructions more closely this time - encountering a slight issue though - after putting something in the cart, not logged in, and using checkout as guest and completing the details, but not clicking confirm order, I can leave the site (close site in browser) and then return to the site and the contents of my cart are still there and when I click on 'checkout' it takes me to the 'completed' checkout page that I closed off from.

Seems that by not confirming the order and leaving the site it is retaining the data, i.e. until I clear browser cache which fixes it obviously - so is this simply browser related?? I wouldn't have thought a browser could retain data from secure checkout page(s)??

Not sure if this error is related to what I just described (or to another occasion when testing whereby I did confirm order but then on the PayPal site I closed off before completing the transaction, at the browser);

Code:
[10-Nov-2019 16:24:08 Australia/Sydney] PHP Fatal error:  Uncaught ArgumentCountError: Too few arguments to function paypalwpp::zcLog(), 1 passed in /home/treeofl3/public_html/includes/modules/payment/paypalwpp.php on line 2034 and exactly 2 expected in /home/treeofl3/public_html/includes/modules/payment/paypalwpp.php:765
Stack trace:
#0 /home/treeofl3/public_html/includes/modules/payment/paypalwpp.php(2034): paypalwpp->zcLog('ec_step2_finish...')
#1 /home/treeofl3/public_html/includes/modules/payment/paypalwpp.php(1890): paypalwpp->ec_step2_finish(Array, 'Yes')
#2 /home/treeofl3/public_html/ipn_main_handler.php(81): paypalwpp->ec_step2()
#3 {main}
  thrown in /home/treeofl3/public_html/includes/modules/payment/paypalwpp.php on line 765
cheers,
Mike