Confusing invoicing process

The order and delivery process went well. But I received a high amount of email notifications from the provider regarding the order, which didn’t bother me in the beginning. What I am extremely unsatisfied of is the payment process. I chose the invoice method and I paid the invoice which was received per email (from MedicalCorner24) and after a couple of days I got a due payment reminder from Shop Apotheke. Apparently MedicalCorner is a contractor and I should have paid another invoice directly to Shop Apotheke. So I had to pay once again and get myself into discussion with their contractor for a cost reinbursement from MedicalCorner. Extremely confusing process and totally unprofessional that the customer needs to sort this out afterwards instead of getting support! My case is still open.