I think the best way is probably just instant dropping it onto the platform then picking it up with fair. This usually eats the shield and is very easy (or forces a ledge slip which is also nice). Naturally you have the suit piece at the end, too, which means you can whack them with a forward toss at the end.
Actually, there's a lot more item stuff to share. I was gonna make a video (I even took a bunch of footage) but it isn't really that necessary... I can just list a bunch of stuff here. None of this should be "news" but a lot of it matters and isn't used much.
1. As mentioned above, ZSS can instantly drop with no bounce, but a hitbox still, an item onto most platforms from a short hop. She can drop a banana on the top platform of BF at the top of her full hop in the same way. You cannot do this with a suit piece unless you are doing ZSS' back jump (flip) animation. You can immediately do an aerial or tap Z again to pick it up.
2. Diddy's shield can easily be poked with a banana from below. This should be followed up with usmash no matter what: if you mess the spacing up and don't poke the shield, the shieldstun and poke from usmash will cause his hurtbox to eventually be exposed to the banana and he'll slip and get hit anyway.
3. In the same vein, you can throw a banana at the front of Diddy's shield on a platform, and then do first hit fair to force an edge slip and reset with neutral B.
3. ZSS can re-catch a banana with a weak glide toss after throwing it. The input is tilt forward->shield->A, then recatch. Note that a banana on shield doesn't give you much advantage and ZSS' grab is booty so there isn't much use for this to be honest.
4. Item tosses in the air allow a brief moment of momentum cancelling. This is most commonly used during recovery, but can also be used like a b-reversal on-stage. You can also throw an item up or down and instantly start moving the opposite direction with almost all of your momentum in tact, then wavebounce, etc.
5. Suit pieces provide 11 frames of shieldstun, which is higher than a banana and most other items. In other words, after a suit piece hits a shield you have 11 frames to follow up (actually a bit more, obviously). You can usually force a ledgeslip on a platform doing this as mentioned above.
6. You can cancel skid animation by picking up an item.