quickest way to add shopify to instagram $5 plan?

Check that Instagram shopping is even available in your country first. It isn't everywhere, and if it isn't, the shop setup just never gets approved however many times you try. Your products also have to fit Meta's commerce rules - some categories get rejected outright.

The route is Meta's Facebook & Instagram app from the Shopify app store. Install it, connect your Facebook business account, let your product catalog sync, then switch on shopping in Instagram once Meta approves the shop. Two catches. Your Instagram has to be a business or creator account linked to a Facebook page, or the connection won't show up at all. And I'm not certain the cheapest plan gets every shopping feature, so check Shopify's plan comparison before you build around it. Approval can take a few days, so don't panic if product tagging isn't there straight away.