Validate checkout form fields with inline validation, not a disabled submit button

Problem

The in-app checkout currently uses disabled submit buttons as a way of validating form fields. Per our guidelines, this is not a best practice.

Proposal

Use inline validation for all form fields and do not disable submit buttons.

• The submit button is active at all times, and the user can click it at any time
• On submit, we validate the form fields and show errors for any fields that are not properly filled out

Designs

See design issue