How can I create an offer where a customer gets a free product when they buy another product?
There are two ways you can build a buy one get one (BOGO) offer. You can create a Post Purchase offer, or you can integrate Bold Discounts with Upsell.
Post Purchase offers can be discounted. This offer shows once your customer has completed their checkout. If they accept, it's added to the same order automatically. Read more about this on our Help Center here: https://support.boldcommerce.com/hc/en-us/articles/20282970543124-Create-a-Shopify-Post-Purchase-Offer
The integration with Bold Discounts allows you to create a BOGO on the Product Page Pop-Up or the Cart Page Pop-Up. Bold Discounts allows you to create a discounted hidden duplicate of any product and then offer it exclusively in Upsell. Read more about this on our Help Center here: https://support.boldcommerce.com/hc/en-us/articles/20394975252756-Upsell-Bold-Discounts-Integration