You may use whatever means of generating an invoice that you choose. Upwork does not stipulate this.
The client may reimburse you for your expenses using the client-side Pay Expenses tool.
The standard Upwork fees apply.
For example, if the art costs $100, and the client reimburses you $100, you will receive $80.
In order to avoid losing $20, you may wish to send the link to the client and have him purchase the artwork himself, and then have him send you the file or the access code.