GTM Postback Tracking For Shopify Website

we are on a MAJOR time crunch and need this done immediately. it's a quick and easy job for anyone who's done it before. we are starting to advertise on a new platform called newsbreak. we setup the shopify tracking but it's not fully working; should have been a 2 second setup....i believe their tracking system has a glitch - the purchase conversion & payment info aren't always triggering, which is a huge problem even when i go to setup ads, the purchase conversion can't even be chosen from the dropdown, even though it's worked sometimes in the testing i believe if we set this up with GTM it should work, and there's other options in the dropdown menu as well. i think GTM would be the easiest and fastest from looking at the drop down. and i believe that's close to already setup for our site.