In fact, send a duplicate as many customers find it helpful to receive two copies, one to keep for their records and the other to send with their payment. Besides invoices, you should send monthly statements listing your customer's payments and all unpaid invoices. You will find some customers may not keep accurate records, even losing or forgetting to record your invoices. Sending statements will also alert your customers that you are aware of outstanding invoices and you expect prompt payment. If you don't, you are inviting exploitation by unscrupulous customers.
Statements are worth the time and postage.
To collect your money, you will find that you must also aggressively manage your receivables with consistent collection activity. As soon as payment is past due, send a copy of the invoice to the customer with a notation requesting their check. When you send out the statement, circle the past due invoices. You will discover that a handwritten note on a statement or invoice is more effective than computer printed messages or past due stamps and stickers. Call your customer asking when you may expect payment.