Hey @PowerMaker, and welcome to the community.
Hope you're doing great!
I understand your concern. Unfortunately, it's not possible to pay a one time payment for Premium for Family. This plan is a monthly plan, which means Spotify is only allowed to charge you monthly.
If you don't have a credit/debit card to pay with - you can always check out the available payment methods in your country by following these steps:
I hope this helps. If you need help with anything else, just let me know 🙂