Send either an money request or an invoice - preferably an invoice
Use Request Money to request payment for goods or services where tracking item detail is not required, or when sending payment requests to multiple recipients. You must enter the total amount to be paid because Request Money does not calculate tax or add shipping amounts for you. When your recipients look at details of the money request, they see your name and email address, the amount and date requested, a subject line, and any notes you entered. When you view a payment request in History, you see the request information and payment information when the request is paid.
Use Invoicing to request payments for goods or service when you want to provide item details, have the tax and discount amounts automatically calculated, and you want to track and manage your invoices and payments. When your recipients review an invoice, they see your customized contact information, payment terms, line item details including unit prices and taxes, shipping and discount information, and invoice totals. When you view the invoice in Manage Invoices, you see the same details your recipient sees, invoice status information, and payment information when the invoice is paid.
Because invoices provide better documentation, this is to your benefit for seller protection. Also, your item must be tangible meaning it can be shipped, tracked or insured if applicable.