is it possible to add a reminder notification for a purchase payment date (note the purchase is in the contract)
Yes, SolarWinds Service Desk (SWSD) supports reminders for contracts, including renewal/expiration notifications, but not natively for a specific "purchase payment date" tied to a contract's payment terms.
From the official documentation and community discussions:
If your payments are tied to renewals or you can repurpose tasks, SWSD handles it reasonably well.