Not all window envelopes are the same.

The USPS has assessed a company $89,000 in additional postage for the failure of the windows on their envelopes. The 1.7 million piece job was entered into the postal stream and during processing windows began to fail for various reasons. Upon further investigation it was discovered the windows were made with inferior polypropylene rather than polystyrene. The polypropylene material is thinner, less durable, and can be prone the hazing in the window rendering the barcode difficult to scan. Make sure you ask your envelope vendors about window material.

See below images of window envelopes made with polypropylene.