How do I give my clients “value” for free or ahead of time when my offers are one-of-a-kind, hand-dyed, silk dresses and handmade-to-order shoes?

I offer free returns, but that is after they pay. I have been trying to come up with a way to give something for free that also makes sense for my business financially. I create a lot of content on social media, and I offer a 15% discount for email signups, but it doesn’t equate to the actual value of my offers. Any advice?