So the clock is ticking and I still don't know which app can replace the original Shopify Amazon sales channel. I have 1700 listings on Amazon already mapped. Is there anything out there can I can install and will understand what is already mapped? I can't relist my items, it will destroy my ratings and ranking.
I setup Codisto in a business where we list on Amazon and pull the orders into Shopify. For listings we keep them separate (create in Shopify, upload via spreadsheet in Amazon) to cater to each platform better, but I believe Codisto has listing management as well if you wanted to do that.
If you're fulfilling Shopify orders via Amazon FBA you can use the Shopify FBA fulfillment service connection (https://help.shopify.com/en/manual/shipping/fulfillment-services/amazon), we use that combined with Order Automator to automate the process (I shared the details here: https://speedboostr.com/automate-order-fulfillment-on-shopify)
In any case I don't think you don't need to relist, you just won't be able to sync listings. If you haven't tried yet, give the spreadsheet upload a try in Amazon (it's under Inventory then Add Products via Upload). If you're regularly creating new products in Shopify that you want to be in Amazon with the same content, you could have a script built that parses your Shopify exported products and puts them in the respective cells in the Amazon spreadsheet for upload.
Couple options there, hope 1 works for you.