USPS | Need envelopes printed? Do it in-house | The Postal Service’s National Printing Center offers competitive prices
Snippet of Article: "...The NPC is the organization’s in-house printer. There are no profit margins or shipping charges on NPC orders.
Ordering in bulk can mean additional savings. For example, 500 No. 10 envelopes printed with a return address cost 13 cents each, including delivery. If you order more than 500, the cost is even lower.
The NPC prints envelopes from..."
Source: news.usps.com