The checkout changes that move the number
Once the leaks are sealed, the highest-return fixes line up neatly with the reasons above.
Show the full price early. Since surprise costs are the single biggest reason people bail, putting shipping and tax in view before the final step removes the shock that kills the order.
Offer guest checkout. Forcing account creation costs you almost one in five would-be buyers over a login they never asked for.
Shorten the form. Baymard's benchmark found the average US checkout shows 23.48 form elements when an ideal flow needs only about 12 to 14. Every field you cut is one less reason to give up. Add the wallets and payment methods your customers actually use, and make sure the whole thing works cleanly on a phone, where more of your traffic checks out than you probably assume.
None of this is exotic. It's the difference between a checkout built around your business and one built around the shopper.