I’m not sure what a “Tag Agency” is, but I’ve always gotten license plates and stickers by mail. Perhaps things have changed here...
I guess it was because we moved from another state that we had to go to the county courthouse (I think it was) and go through that ordeal. When you renew your plates, it is by mail.
Tag agencies are private businesses that contract with the state, and there are numerous locations. You can get your plates and renew drivers’ licenses (unless you are from out of state again!) at any of them. Since they take most of the burden off the DMV offices, it is much easier and faster when you do have to go to the DMV. Also, probably for that reason, the DMV folks here are nicer and easier to deal with than the crabby folks at the TX offices.
Still hoping that TX will secede and take OK with it! ;-)