Adding and removing products from collections in Shopify is very easy using Airtable. To start, ensure that you have an Airtable sheet to which your products are being synced to/from Shopify, and an Airtable sheet to which collections are being synced to/from Shopify. Now, link the two sheets, and voila. If you add a product to a manual collection in Airtable, and run the Airtable to Shopify sync, then that product will be added to the collection in your store.