Add-on doesn't seem to be installed successfully / I never get the "addon-installed successfully" popup: This issue can occur when you sign to Google accounts, and you are not using the principal account for installation. An easy workaround is to open a private window, sign-in to your Google account, then proceed to the installation.
After successful installation, Merge for Gmail is not visible in Google Sheets: Simply refresh Google Sheets, wait for 10 seconds, and it should appear in the menu !
Account & Billing
We have a dedicated page about account management, billing, subscription cancellation and how to get invoices here : Account & Billing