Generate invoice number

Deprecation notice: The /v1/invoices endpoint is deprecated. Use the /v2/invoices endpoint instead. For details, see PayPal Invoicing Basic Integration.

Generates the next invoice number that is available to the merchant. The next invoice number uses the prefix and suffix from the last invoice number and increments the number by one. For example, the next invoice number after `INVOICE-1234` is `INVOICE-1235`.