Then they cancel the return and you have both phones. Typically with online purchase returns, you have to ship it with your own box. You should have gotten a return label when you clicked on that return button and also should be in your e-mail.
OP, you’re either trying really hard to find a way to game the system or really not understanding something that is kind of basic credit card consumerism.
Collections means that you will be contacted for money that you owe by a certain department - usually called Collections.
If you still don’t pay, then a number of things can happen, up to and including a judgment against you for the amount owed plus court fees.
All that said, Apple will most likely attempt to charge your card first. If you try to dispute or block the charge, you’ll lose. If you make it so the card is no longer valid, the collections process will begin. Either way, the phone you kept will most likely be blocked in some manner, making it an unusable device. Apple may even block the new device as well.
There really isn’t a win here. Return the phone and stop thinking shifty thoughts.