Complete (80%+) lowers have to be shipped to an FFL licensed gun shop who receives the item and does the transfer and any background check for a small fee. Your online seller needs a copy of your local gun shop's license before they can ship, but most seller websites have a searchable list of the gun shops they already have paperwork on. It's easiest to use one of them if you can find one with a reasonable fee.
If your local gun shop you want to use isn't on their list then you have to ask them to fax over a copy of their license to the seller. Then to be safe you should contact the online sell to confirm the license and shipping address so the item can ship. Sometimes this take forever so it's better to just use one on the list if you can.
The gun shop I use only charges $15 for FFL transfers, but some charge as much as $50 in an effort to get you to buy local. If you find a good and friendly local shop that doesn't overcharge, make sure you give them some business when you can as they are dying off fast.