what version ?
version 7.1 had a paid mode that basically got you tech support. the company that wrote that version of eudora and provided the support is no longer in business.
You can try this code but i dont know what it will get you other than it saying paid
eudora 8 or eudora ose is open source code based on thunderbird and its free. i dont believe that there is a paid version
basically there is nothing left to pay for in eudora. Its free
if you need a download link i can provide it