Inquiries about purchasing

Hi, I am a well-used user who purchased about your service a few years ago. Additional purchases will be made for higher-level services.

Previously, I am using [Self-service] and I would like to upgrade to [Self-service with Publishing] or [Full-Service with Publishing]. Do I have to pay all the new money? Or does it deduct the existing purchase amount?
Can I add only the plug-in I want after purchasing it with an additional amount?