Nice Anodized Aluminum case painted black for Raspberry Pi 3B+/3B motherboards.  It comes with packing material in a plain white box so you can put your custom label on.  Four screws hold the unit together, and inside a mounting screw holds your Pi board securely in place.  The aluminum gives great heat dissipation for a fan-less operation.  One time flat rate shipping of $5, no matter how many units you buy.  Make an offer for 25+ pieces.