We use the Staples Double-Window Gummed envelopes because of the value and durability of the product. We have changed all our invoicing purchase orders and other letters to match the envelopes because of their durability and value for the low cost. It makes sense from a cost standpoint to use one simple product for all situations which means we stock fewer items and that equals lower costs. Simple use means more profit in our company which is a win for all. Small savings from expendable items spell success.