You can make it as short and sweet as you want, or complicated.
If someone is going to want an invoice, this is how I do it.
Why? Because of US tax laws.
When I get a Misc. 1099 form showing the taxable amount, there is always some applicable deductions -
Cost of Domain Name
Cost of Hosting
Cost of Advertising
Commission
Capital Gains (or Losses)
Having the invoice that clearly shows who gets what, who pays who, and what I ultimately end up with is crucial to illustrate.
You have to realize that the buyer most likely will be filing taxes on his end showing this purchase as an advertising cost, a deduction on his end.
Any inquiry (or audit) into their expenditures will surely show them paying for the domain and the sale can be traced back to you.