Yes, you can set-up LetsBuy with a manuel way. If your Shopify store does not connect to LetsBuy App (small percantage of Shopify Stores are not able to connect Shopify Apps), it's really easy to make the set up.
You just add <script src="https://letsbuy.io//shopify.js?v=1&shop=shop-name.myshoify.com"></script> just before the end of the <body> tag of your Shopify theme and change the shop-name with your Shopify store name in this code snippet.
It's that simple! After you add this the script, you can continue to open an account and use LetsBuy.