The United States Postal Service has no regulations that can directly prevent you from using an envelope for sending something else rather than the intended one. It is the same for a bill envelope. You only need to pay for the required postage and they will successfully send the envelope to its destination. The most important thing that you need to keep in mind is that you need to cross out the old stamp and put on a new stamp over the previous one. However, if you need to send your regular QuickBooks checks, manual business checks, or personalized business checks, you choose the more secure #9 Security Tinted Window Envelopes.