Say for example im selling "custom artwork" or something. After a customer completes payment, they get sent to a questionnaire to give details about how they want their "artwork" done? Kind of like Fiverr in a way.
Side question: Whats the best way on shopify to send out invoices to customers after "artwork" is completed? I plan on selling custom artwork, and then wanting to have the option to send invoices for any additional revisions and add ons.
You could modify your confirmation email to add the questions or create an online questionnaire such as through Google Forms which you link to in that email.
In terms of your second question you can create a new draft order for the customer and email them the invoice: https://help.shopify.com/en/manual/orders/create-orders