Offering discounts is a great way to incentivize a purchase. You can offer your customers a monetary, percentage, or shipping discount for collections or specific products in your store.
The most popular promotions are "Buy 2 - Get 1 Free" or "Buy 3 - Get 2 at 30% OFF", etc.
Offering a free gift to an existing order is one of the oldest sales tricks out there and it works great both offline and online.
You can do the same by using one of the Shopify apps or some even do it manually but it won't look as good as in the examples above.
NOTE: Do not forget to inform your e-shop visitors about the available discount offers directly on your product page, by creating a separate discount page, or by promoting it in various marketing campaigns.