noun
window envelope; billing envelope
An envelope with a transparent window, commonly used for invoices, statements, and business mail.
請求書は窓付き封筒で送られてきた。
The invoice came in a window envelope.
封筒 is a general envelope; 窓付き封筒 specifically has a transparent window.
Compound of 窓 (window) + 付き (attached, with) + 封筒 (envelope).