156a vanilla install w/demo data
drop demo data
import db from live backup
rerun zcinstall
install zca_bootstrap
change template back to classic responsive.
restore original includes/classes/order_total.php that zca_bootstrap overwrote

Setup payment modules
AIM setup but disabled
SIM setup enabled
Square setup enabled
PayPal setup enabled

setup USPS shipping: remove/reinstall & enable
setup FEDEX shipping: remove/install & disable

add item to cart
click checkout
white screen

myDEBUG
Code:
[09-Jan-2019 10:33:25 America/Los_Angeles] PHP Fatal error:  Uncaught Error: Call to undefined method ot_coupon::collect_posts() in /home/cPanelID/public_html/156/includes/classes/order_total.php:162
Stack trace:
#0 /home/cPanelID/public_html/156/includes/modules/pages/checkout_payment/header_php.php(100): order_total->collect_posts()
#1 /home/cPanelID/public_html/156/index.php(36): require('/home/cPanelID/...')
#2 {main}
  thrown in /home/cPanelID/public_html/156/includes/classes/order_total.php on line 162

[09-Jan-2019 10:33:25 America/Los_Angeles] Request URI: /156/index.php?main_page=checkout_payment, IP address: 98.146.xxx.yyy
--> PHP Fatal error: Uncaught Error: Call to undefined method ot_coupon::collect_posts() in /home/cPanelID/public_html/156/includes/classes/order_total.php:162
Stack trace:
#0 /home/cPanelID/public_html/156/includes/modules/pages/checkout_payment/header_php.php(100): order_total->collect_posts()
#1 /home/cPanelID/public_html/156/index.php(36): require('/home/cPanelID/...')
#2 {main}
  thrown in /home/cPanelID/public_html/156/includes/classes/order_total.php on line 162.
there are other DEBUG-adm but I think they're all related to email not being setup

Add'l info but probably not related
FREE SHIPPING is a selectable option even though the order total is greater than $0
FREE SHIPPING was removed and reinstalled