The option is offered in the Condition Type dropdown, but in the current version the background job that evaluates conditionals only knows how to check email opened/not opened, email clicked/not clicked, SMS delivered and SMS replied. A conditional set to A purchase was made is removed from the queue without sending either branch, so customers reaching it simply stop there. Until this is completed, use the email/SMS conditions, or target buyers with campaign segments such as Checkouts in the Previous week instead.