Add to cart is not missing, but in fact invisible on the Playstation 3 Store. If you select overview, then navigate up, you can still add things to your cart. (Bug/Workaround) :
Ys: Memories of Celceta ** European Version** Sony PlayStation PS Vita Cart Only | Being Patient
Add to Library vs. Add to Cart - what's the difference? : r/PlayStationPlus
Backstage Equipment 8-Crate Set Box Platinum Cart SB-08 PS B&H
PlayStation - Get 10% off one total cart purchase when you... | Facebook
Sony $60 PlayStation Store Card [Digital] Sony PlayStation Store $60 - Best Buy