You might try different link-shorteners to see if any of them don't passthru the preview image/info we provide for Facebook shares, or of course you could just remove the FB link preview by clicking the X on it before you post. You can also share your own image on FB and just add the link and any text you wish to that, so your image would be shown instead of the link preview.
You could ask the merchant to upload a logo and/or product images, so that would be shown instead of the generic cart icon in those FB share previews.
They can upload a logo in Seller Admin under Manage Seller Account > Edit Preferences (this would be shown in the preview for your Common Hop Link), and/or upload a product image in the settings of each product (which would be shown in the preview for your Product Hop Link for that item).