If you can find it for $15.18 without having to buy other services, that's a deal and you should take it. Most discounts like that are offered as loss leaders in the hopes that you'll buy other add-ons.
For a customer it's irrelevant that the discount is a loss leader. The end result is that I can renew a .space domain for half the price at Namecheap, without having to buy other services.
Understood... personally, I'd rather pay the extra dollar or two in order to NOT have to click through promotional crap during registration. I don't know about cloudflare's process, but I'm assuming it'll likely be as clean as google's (by comparison).