What is the ruling on variable and undisclosed shipping costs for sea freight—which cannot be refused—for goods purchased online, knowing that shipping fees are required to be known? Are they considered known given what was mentioned, or do they fall under the general affliction (umūm al-balwā) and thus become permissible?

This transaction between you and your client's warehouse is a permissible and valid rental (ijarah). Its validity does not require knowing the shipping cost when purchasing the goods. Rather, the essential condition is that the shipping cost be known when agreeing with the shipping provider, because one of the conditions for the validity of a rental contract is that the rental fee (ujrah) be known.