We rebuild the page closest to your sale and run it against the one you have now, on your own live traffic. The scoreboard is revenue per visitor, not a dashboard metric.
Usually the product page, because that is where the money is decided. Copy, layout and offer rebuilt as one considered change, not a list of tweaks.
Old page and new page live at the same time, your traffic split between them. Seasonality and ad spend hit both equally, so the result is the result.
Two or three tests a month after the rebuild, each one queued from what the last test showed. Fewer opinions, more tested changes.
If the rebuild does not lift revenue per visitor, you keep your money.