Cal here from Shopify, thanks for reaching out to us.
You could start using Discount Codes, and leverage the Buy One, Get One discount type. We have a great guide on these types of discounts here. If a customer purchases X, they get Y off. You can set a minimum quantity, to cater for one of your offers.
Here is an example of someone buying two products, then getting 1 item 20% off:
You would create another for your 30% off offering too:
Let me know your thoughts when you get a chance to test these codes! If you were looking to avoid using discount codes, you would need to look at third-party discount solutions in our app store, that can be found here.