I highly suggest you search for, download and read the PDF for inbound FBA shipments as well as the associated fees so you don’t get surprised.
Boxes can be “oversized” but still acceptable to FBA, you just pay higher fees. All multi-pack boxes (more than one unit per box) must be under 50 lbs or you run the risk of losing your goods and being blocked from shipping into FBA. If your single product is over 50 lbs you need to put a heavy, team lift sticker on the box.
Again, do yourself a favor and download the FBA PDF document for inbound goods and make sure you understand the associated fees. Its very detailed and will save you a lot of headaches (and money). If you are sending in pallet quantities, check out Amazon partner LTL costs for shipping. You can save a ton of money in freight by using LTL versus UPS.