Instrument the funnel
If you only look at purchases and CPC, you cannot see where money exits. Track product view, add to cart, checkout start, shipping selected, payment started, purchase. Look at them by device and by campaign.
A campaign that “converts badly” may be delivering the right people to a payment step that fails on mobile Safari. That is not an audience problem.
Surprise costs and forced accounts
Shipping and tax that appear late train people to abandon. Show a realistic total as early as you can. If you cannot calculate it yet, say what remains unknown instead of pretending the subtotal is the price.
Forced account creation before payment is another classic leak. Offer guest checkout. Invite the account after the order when the buyer already trusts you.
Payment and performance
Payment iframes, fraud scripts, and tag managers stack until the button feels dead. Cap what loads on the payment step. Test on mid-range phones on cellular, not only on office wifi.
Error messages should preserve the cart and explain the next action. “Something went wrong” after a declined card is how you pay for the same click twice.
Fix before you scale
Every abandoned checkout on paid traffic is a fee you already settled with the auction. Raising budget before you close the leaks buys a larger fee.
Run a weekly review of step drop-offs with marketing and store engineering in the same meeting. Shared numbers beat separate dashboards that never argue with each other.