Seller contact information: This not only lets your customer know how to contact you if they have any questions about the invoice, but could also lead to future business; if they are happy with their experience working with you, they might use the information to contact you again or refer you to a friend.
Product or service details: Specify Description, Quantity, Rate, Amount and Subtotal. The more details you include on the invoice, the better. This way, your client will know exactly what they are paying for upon receiving the invoice. Make sure that your client can understand every item, so that as soon as they receive the invoice, they can pay you instead of asking questions and delaying the process.