Meta charges you based on the number of messages that you send in a month. These charges are separate from your Spur bill.
Since Spur is an official Meta partner, you would need to make these payments directly to Meta. Spur charges no additional fees or commission to these payments (*cough* unlike some competitors *cough*) as a result of this direct partnership.
Payments for your messages go directly to Meta. Spur plays no role in how much those charges are and the payment methods offered to you.
At the time of writing, Meta only excepts payments through card.
You can login to your FBM and checkout your analytics to see your charges. To learn more about these charges, you can read this article.
To setup your payment method with Meta, you can go to Settings > WhatsApp > Go to Payment Settings (under Payment Setup)
To anticipate the amount you need to pay Meta based on your conversations, we also have this handy calculator: https://www.spurnow.com/tools/whatsapp-pricing-calculator