Is the annual paid subscription to a shipping service that offers advantages such as weight calculation per 100 grams and a 20% discount for large weights permissible, and what is the ruling on shipments made through it? And do the rulings on discount cards, open buffets, or minor Gharar apply to it?

It is forbidden to subscribe to the shipping service in exchange for a discount, as it involves Maysir (definition) (gambling/unlawful gain). The money paid for a potential discount represents a certain loss and a potential gain, which is a type of discount card forbidden by Islamic Fiqh Councils. There is no blame for what was shipped before you knew of the prohibition, but you must stop subscribing to this service.