Do NOT buy Delta tickets on Westjet flights!!! Westjet operates those aircrafts and sells those seats in bulk to Delta, as "codeshare". You will pay Delta but check in with Westjet.
I made that mistake a few years ago. Let me tell you my story.
I paid for seat selection and checked in early. I made sure all the rules are followed. At the airport, Westjet suddenly told me that somehow my chosen seats were broken, out of the entire sold-out plane. They said they needed to bump us but there was no flight until the next day. They said since we paid with Delta, they could not help us so I have to deal with Delta for rebooking. My infant child did not have enough formula in the carry-on luggage to last overnight and could not just eat airport restaurant food... but they did not care at all. They were kind of, too bad so sad. Their position is that you are not their customer!
This is the moral of the story. When you pay Delta to ride Westjet, you will be bounce around like a tennis ball when there is an issue, with each side blaming each other. Westjet does not consider you as their client. They will bump you first, if they need seat, even for a crew flying back. They will make up excuses such as mechanical failures, but just on those specific seats that you get. When you complain, they will ask you to walk to the other side of airport and deal with Delta. With Delta, they will claim it's beyond their control and if you need to rebook, then you will not have the same direct flight and you will go through an American city such as Detroit or Minneapolis as those are Delta's hubs. You will end up losing hours, if not overnight, to be rebooked.
Yes, I compared and saved about $200 for a family of 4 booking through Delta... but it was a nightmare. I would not do it again. Luckily, we were not dragged down by force like United Airlines. However, they definitely picked up first to bump.
By the way, they found seats for us after our relentless complaint and threaten to go to media on how airline bump infant and cause food shortage. So we were allowed to board... Guess what?! We had our exact same seats... nothing was broken. I guess they made that excuse, but decided to bump other people instead. I bet it was done so some pilot or flight attendants can go home early instead of waiting for a later less busier flight!