Welcome to the Seller Community @Sassycat!

This Support Center article has the steps to send an invoice from your online Square Dashboard or from the Point of Sale app. You'll need your customer's email address to hand to send the Invoice and that's about it! You can add items/a custom amount and request a shipping adress or allow your clients to save their card on file in a few clicks. 

If you send invoices frequently you can also edit and save default settings to speed up the process.

If you have any questions please let me know!