Single Monitor Arm 13-34 Inch, Adjustable Gas Spring Arm, Monitor Desk Mount for 4.4-19lbs, VESA 75/100mm with C-Clamp/Grommet Base, Ergonomic Stand for Work/Gaming

This is a decent deal for a single monitor arm at $21, especially considering it supports a wide range of monitor sizes and weights. While the original price and discount percentage are not specified, the current price point is competitive for this type of accessory. However, AliExpress can have variable shipping times and product quality, so it's important to check seller reviews.