E-Commerce UX Questions

I have a few questions in my mind and stumbled across stack-overflow UX, was hoping your inputs would help me in my new application. Application is an multi platform e-commerce.

1) I'm facing an issue with OTP being bit pricy for my startup.. Should I Only OTP a person upon his first purchase?

2) Should Sign/Signup be prompt'd upon first time in app? or only when customer starts ordering? to keep UX.