Very often, even if you choose slower shipping in order to gain the credits, you'll still get the item delivered according to the faster shipping schedule. Unless it is crucial that you receive the items at a specific date, this seems like a good option.
Besides using portals and getting gift cards at bonus category locations, consider shifting around your delivery schedule. I'm not going to ask what items you're purchasing from Amazon, but if they're not extremely perishable, you could make bulk order purchases starting next month (October) using your Discover card to get the bonus category then.
If you haven't already taken advantage of such, there are some offers that you can use. For example, there was an offer for a discount if you used your Discover cash back towards an Amazon purchase. I'm not sure if that's still active, but you might want to look into it. Make sure to always check if there are any coupons that you can clip to cut down on prices.
