There is a bug that make it harder (i would even say - impossible) to play - quite often push wouldn't actually push the package. Even though hit comes to the middle.
Very solid idea, a bit of work and it would be a very satisfying experience.
Things may imporve this game are - first of all making conveyor like two times slower. Need to test to check how it feels, but feels about right for the first jump of testing. It should fix the issue of your robot moving with about same speed and player being unable to react to parcels of different color when they are coming together. It also would make sorting 'trash' more fluid, since you need specific position for it so it won't hit anything. Though, personally, I would also make wider 'side lanes' and wider 'trash disposal area'(or shorter parcels) so it won't feel so restrictive to push parcel 'just right'. Slight alowance for a push is a good idea here, since you always will be pressured on time/position even with slower speed of the conveyor. There are times when I could push parcel slightly off the lane, but most of the time it would just disappear (as I understand - 'trashing' mechanic in place).
Also would be a nice, of course, to have 'health' for mistakes so player would see how many they left.
Its really crutial to sort the bug with parcels not being pushed, of course. Sounds and particles would be nice, but they are not as important to make game feel good.
Also would love to have spacebar for pushing and esc->restart for restart instead of space being restart button. But that is prefference issue and also not as important.
Keep it up, though. Working on good ideas is satisfying :)