If you have an AmEx card that earns unrestricted MR points, even a no annual fee Everyday one, you can set your Rakuten account to earn MR points instead of cash and a decent number of times Rakuten will be a better deal than Delta's shopping portal. It doesn't matter which card you actually pay with from the Rakuten portal as long as the MR earning card is active.
I typically pay for transactions made through Rakuten with my Delta Platinum Skymiles card but save up the MR points generated via Rakuten until there's a 25% transfer bonus to Flying Blue, which generally happens for a month or so every year, rather than transferring to Delta at a 1:1 ratio.