Actually there is a legal problem here- if you are not an employee you must be able to provide an invoice, which most of us cannot.
The trick is: who do you send the invoice to, and will they ever pay it?
I've invoiced companies that made me do unpaid projects and while it makes me feel better it has never once gotten me paid.
The invoice is not the problem, handling taxes is. In most countries (I'm not an American) an Invoice means that you are self employed and that you should pay taxes on that part as self employed.
There are services around it that charge percentage of the transaction and I'm not sure will handle such small amounts.