$10 price drop. Amazon has the Sony Playstation 4 DualShock 4 Back Button Attachment (2020 model) on sale for $20 with free shipping on $25+ or with Prime.

The attachment adds two programmable buttons and has an OLED display and pass-through audio jack.

Other retailers have it on sale for $20 for Green Monday:

Best Buy (store/curbside pickup)

GameStop (store/curbside pickup)

Walmart with antonline / antonline.com (free shipping)