Official comment
You can do this by selecting "Allow buyer to change quantity" and having the default quantity at 1 or zero:
You do have to list both products on the form. Some of our clients will have a primary item with a quantity of 1 and then a secondary product with a quantity of 0, with a note to increase the quantity if they want the second item.
Alternately, for add-on products, you can use the upsell form feature in ONTRAPORT Pro or Team accounts.
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