Creating BOGO, Discount, or Buy X Get Y offers is incredibly easy with In Cart Upsell and Shopify's new improved Discount Codes.

In this article, we'll create a sample offer for a free Toaster when you buy 2 Mugs.

STEP 1 - CREATE DISCOUNT CODE:
In your store's admin area, go to Discounts -> Create Discount. Since the customer won't be typing the code in, click the "Generate code" link to have Shopify automatically make one. Then under Options -> Types, choose "Buy X Get Y":

In the "Customer Buys" section, we set the condition to 2 Red T-shirts:

In the "Customer Gets" section, we say 1 Shorts:

Copy the discount code to the clipboard - we'll use it in the upsell offer.

TEP 2 - CREATE UPSELL OFFER:
Create a new offer. Then add the product(s) you wish to offer. The product(s) should be the same as what was earlier added inside the "Customer Get" section

In step 4 of the offer builder, add a rule that the offer should appear whenever there are at least 2 T-shirts in the cart

In step 5 of the offer builder, set the discount code to be the code we created at the beginning:

Then save the offer.

Of course, you can customize the offer in any other way you like - we've skipped over the other steps in this recipe in order to focus on using Discount Codes.