Investigate adding SPB to the Pay for order page

[james-allan]

Describe the bug

Slack: p1619737681069700-slack-C7U3Y3VMY

Investigate if there are technical reasons why we don't display the Smart Payment Buttons on the Pay for order page.

To reproduce

1. Manually create an order from the admin dashboard using any standard WC product.
2. Click the pay for order link. https://d.pr/i/NAGlaT
3. On the "Pay for order" page, note that there's no PayPal Smart Buttons, just a Pay for order button which takes you to Paypal.

Screenshots