I would like to create a coupon for the basket dropouts (5€ extra). This coupon should be combined with other coupons. e.g. 15% on everything and 30% on sale. These two coupons (15% and 30%) on the other hand should not be combined with each other.
Is that possible?
Unfortunately magento doesn't support more than one coupon code on a cart/order.
I'd recommend checking the Magento Marketplace though. It looks like there may be some solutions out there that could help with this, e.g. https://marketplace.magento.com/amasty-coupons.html (this is not a recommendation, I have not used this extension).