A checkbox can make a purchase before the shopper does.
The customer chooses a product, reaches checkout, and finds an optional service already selected: insurance, a donation, a warranty, premium delivery, a membership, or some other paid addition.
Nothing dramatic happened.
The interface simply treated failure to notice as a decision.
The Federal Trade Commission has repeatedly identified pre-checked boxes and preselected options as dark-pattern tools when they steer consumers into purchases or recurring billing they did not affirmatively choose.
See the FTC’s dark-pattern report.
Defaults are powerful because most people follow the path
A default is not neutral merely because it can be changed.
Checkout is already a high-attention task. The customer is reviewing an address, payment method, shipping speed, taxes, discounts, and the final total. A small preselected option can survive precisely because the shopper is concentrating on something else.
FTC consumer guidance on negative-option subscriptions explicitly warns people to look for pre-checked boxes that can enroll them in future billing.
See the FTC’s free-trial and auto-renewal guidance.
The agency’s current fee guidance makes the principle even clearer: a supposedly optional charge may effectively become mandatory when it is included through default billing or a pre-checked box and removed only if the customer notices and challenges it.
See the FTC’s Unfair or Deceptive Fees FAQ.
Optional should mean chosen
The clean design is uncomplicated.
If an extra service costs money and is not necessary to complete the underlying purchase, leave it unselected.
Explain what it does, show the price, and let the customer choose it.
That changes the meaning of the transaction from we added this unless you object to you added this because you wanted it.
The distinction is small in interface terms and enormous in consent terms.
A customer should not have to audit a checkout page for financial decisions made on their behalf.
An optional purchase should begin with an action.
Not with a trapdoor.