I use about 1500 #10 Left Window Envelopes a year. The only problem I incurr is sealing. The cost factor of switvhing to "PELL AND SEAL" ENVELOPES IS THE COST.
They are a GREAT value for the price. However I'd like for them to be a bit more secure. It's still possible to see through them. Not completely; one would have to work at it. But since I use these primarily for business it would be nice for a tad more security.