Is it Islamically forbidden to add the cost of administering IV fluids and cannulas to the price of treatment and fluids without the client's knowledge, or by informing him of the total amount without itemization, especially if some clients do not pay the administration fee later on?

It is not permissible for a pharmacist to take a fee for inserting a cannula or administering solutions unless the patient is aware of it. This is because a lease (ijara) contract requires knowledge of the fee for its validity. The patient often assumes that this act is a donation from the pharmacist, and the pharmacist is not considered someone who deserves a fee even if it was not agreed upon, because it is not customarily known for a pharmacist to perform these actions for a fee; rather, the default is that they are done gratuitously.

If the pharmacist wishes to charge a fee, they must declare it. Otherwise, the default is that they are acting gratuitously, and it is not permissible for them to add a fee without the patient's knowledge. Jurists have explicitly stated that whoever performs work for another without a fee (ju'l) receives nothing, unless the worker is prepared for such work for a fee, like a sailor, a cupper, or a tailor, in which case they receive the customary fee (ujrat al-mithl). As for the pharmacist, if they do not declare that they will take a fee, they are not entitled to one, because they have not prepared themselves to take a fee for these actions.