How do I give customers their requests when I'm not Co-Author?

Message me or the Co-Author the link to the poster. [VALID LINK PLZ]

If a customer requests for a poster at my page and not at the shop, am I still required to complete the request?

Yes. As long as when you MESSAGE me the poster link, you include the whole request form they have sent you on your page. 

[more will come]

Message me if you're question is not answered.