Fixing Discrepancy in Reported GA4 and Shopify Reporting

[vintageart]

I have a -20% discrepancy in reported sales between Shopify and GA4.  Any suggestions as how to trouble shoot.

I had been using GA4 and UA in parallel for over a year in anticipation of the retirement of UA.   GA4 was implemented using Google Tag Manager. Sales and ATC data tracked.   When Shopify FINALLY introduced their integration in March I retired the GTM work-around and deleted custom tags (ATC, etc.).   Reported GA4 revenues ane 20% below those reported in Shopify Analytics.  Any suggestions as to root cause and trouble shooting?