Massive Disk Pro is billed through RevenueCat, so managing it doesn't happen on this website.
Use the manage-subscription link in your original purchase receipt email, or open the RevenueCat customer portal with the email address you subscribed with. Cancelling stops the next renewal — you keep Pro access through the end of your current billing period.
Install Massive Disk on the new Mac and it will pick up your subscription automatically once you're signed in with the same Apple ID, or you can enter your account ID (the MD-XXXX-XXXX-XXXX code from your purchase) from inside the app's settings.
For refunds, billing questions, or anything the receipt link doesn't cover, contact support with your account ID and we'll take it from there.