Bookhead can sync non-book items (like merch and sidelines -- any product without an ISBN) to Squarespace or Shopify.
This works with Booklog and IBID. It's untested with iMRCHNT.
We currently don't support image upload for non-book items, so you'll need to continue uploading the product images to the sales channel platform. If you are interested in a Bookhead-based image management interface, then let us know in this feature request: https://bookhead.canny.io/feature-requests/p/manage-non-book-product-images-in-bookhead-staff-interface
This feature includes some logic that links your existing products with these items, so please let us know if you encounter unlinked products!