Should I offer lower product costs on my Shopify store or free shipping?

I've found people tend to prefer free shipping, even if it means paying a bit more for the items. That way, it's not a surprise when they get to checkout.

The number one cause of abandoned carts is unexpected fees during checkout. Regardless of what you decide to charge, don’t wait until the final checkout step to surprise people with the cost.

Marketing a flat rate or free shipping is better than lowering your product cost because people like to know what they are getting into before adding products to the cart.

Definitely free shipping.

Many people will abandon their carts because they've come to accept what a product will cost and then they are surprised when they find out it will cost more because of shipping. You can, of course, also offer free shipping only above a certain order value. This encourages customers to spend more on your store so that they can cross the threshold for free shipping.