Question about Trade In method

Hi, I am planning to buy a iPhone 11, 128 GB, White. And I currently have a iphone7Plus, in good condition.So if I adopt the trade in payment method, when do I need to send you the old device? I mean, I only have one phone, if I send you my old phone without getting a new phone at the same time, I will not have a phone to use for several days, and that would be suck.

Looking forward for your reply , thanks