Hello Alexeik,
We apologize for the inconvenience caused, as per the description above, you are not able to combine PDF files as it's asking you to repurchase the services, Is that correct?
The record shows that you are subscribed to Adobe PDFpack services with your current Adobe ID and password. You can access the service through URL https://cloud.acrobat.com/combinepdf with your Adobe ID and password on a browser like Chrome, or Firefox, or Safari.
And, the "Add" button is a designed behavior in Combine FIles tool, and it cannot be changed in "Open".
If you are using the freeware Adobe Reader DC and getting the repurchase error message, please sign out and sign back in Adobe Reader DC using your current Adobe ID, then check for any pending updates from help>check for updates, reboot the machine and then try combining files.
For more troubleshooting, you can refer to Troubleshooting Create PDF, Export PDF, Adobe Send, Fill & Sign … services(PDFPack or Document Cloud Services).
Let us know how it goes and share your findings.
Regards,
Anand Sri.