Interaction Design

What are some of the best practices for implementing magic links?

Onboarding order: ask first to link Twitter account, or ask for creditcard information?

What is the quickest way to jump to mostly functional iOS ecommerce app (With a good design) that sold like 10 things max?

Any examples of "Find or create" input fields?

Payment during signup process or just before using the key feature?

Anyone have example "style guides" for a website / brand? (e.g. logo guidelines, icon set, typography, etc)

From a UX/design perspective, what are some alternatives to having a popup/modal for email opt-in?