For a 50 piece or more per shipment you can choose Parcel Select. This option is available today on the Amazon.com website, however, Parcel Select is still being worked out on the eBay website leaving those customers little option except to use another service like UPS ground. If you've got your own shop and want to offer USPS select you'll need to contact the postal service. And from some reports information varies from Post Office to Post Office---even their own staff is confused and uninformed of the rules. The USPS Website provides limited information too. There are no links to assist businesses to come on board with Parcel Select. It's almost as if the USPS doesn't want that business and is advertising for UPS.
A quick comparison of rates shows that UPS may be a better solution too. I used Amazon's handy postage calculator to do a comparison of shipping packages of different rates and sizes from Northwest Indiana to Virginia this morning and this is how rates compared:
So it looks like it pays to ship packages 10 lbs and over UPS. Add the free insurance that you receive on some UPS shipments and it looks as if even if you can go the Parcel Select route it really isn't worth the trouble. It's a great time to rethink your shipping especially if you've been using Parcel Post---an option no longer conveniently available.