Eunice here from Shopify. Thanks for posting and welcome to the Community!
We have many apps available in the Shopify App Store that can help with this. The Free Gifts BOGO buy x get y by Secomapp would be a good match for your needs. It does start from $29.99 USD a month, but, I love that it gives you the freedom to customize your offers in a way that most other buy-one-get-one-free apps don't. Such variations are as follows:
If you're looking for a cheaper alternative, I recommend checking out the app Ultimate Special Offers by Pixel Union. At $9 USD per month, this app will allow you to create buy-one-get-one-free offers.
If you'd like to learn more ways to create special offers and discounts for your business check out this great blog from Shopify.
I hope this helps! If you have any questions please sing out here.