100 rolls? That's going to cost you a pretty penny to ship! Considering each roll weighs 5.4 oz you're going to have a package that weighs more or less 34 pounds! Your best bet would be to get a large flat rate box at the USPS store, the maximum weight for the large size is 70 lbs.
Yes. I would ship 50 in each box. I would seal them all in a plastic bag and use lots and lots of bubble wrap and tape.
Pack in a medium size box. Well wrapped and addressed to you. Pack the medium box in a large box and extra crush material around that with cardboard. Give USPS room to beat up the corners while keeping the inside box intact. Use plenty of tape on both boxes.
That's the perfect application for Flat Rate Box. Heck, you could line it with sheet metal if you're concerned about abuse. But @rte has it right. Except, for "crush material" I'd be tempted to use just more layers of cardboard -- it's heavier, but that doesn't matter, and it'll give you stronger protection.
A flat rate postal priority box is what you need to lower your shipping costs. You can use tape to help strengthen the box. You can also put the coins in a box, seal it, and place that box in a flat rate box. Do not tell as anyone at the post office what’s in there and make sure they don’t rattle or make any noise. If they are in plastic tubes, stuff the tubes so they don’t rattle. And ship with insurance.
