The cheapest growth test I ran recently cost $0 and one awkward phone call (I will not promote)

I kept tweaking ads and landing pages for a little product I run, and nothing moved. So I called five people who'd bought once and never came back, and I asked one question: what almost stopped you from buying? Three of them said the same thing, they didn't trust the checkout page, not the product. I fixed one line on that page and the next week was better than the previous three combined. If you're stuck, skip the next tool or tactic for a day and ask the people who already paid you. What did your last five customers almost not buy because of?