I don't think it is unethical, to accept the deal that Doordash for some reason decided to offer. I mean you'd have to read the fine print I guess since you have a relationship with Doordash as the customer using their app, which probably has some contract.

It may be possibly ethically questionable to intentionally post a menu that Doordash's automated system will misinterpret (which would clearly be possible in this case, since they figured out that Doordash was charging the plain slice price for the custom one, so they could just drop the plain slice price to $1). I mean, again they didn't ask Doordash to start this relationship and they don't have any contract, so... I guess their obligation would be to honor the $1 plain slice for anyone who wants it. Maybe make it a $1 no-cheese, no-sauce slice.

Another option would be to spread word about the community that Doordash offers a $8 discount on your pizza. Like, if you tell the person "hey, order on Doordash to this address, they'll give you a $8 discount and we just have to have a dude come hand the pizza off from me to you" right before they get to the register... seems pretty ethically clean to me, you are just informing people of an offer you heard about, you have no contract or relationship with Doordash, right?