I'm a bit rusty, so someone check me on this, please.
To destroy a Customs Office outside High Security, you need to shoot it until the shields drop to 25%, at which point it goes into Reinforced Mode, like a Player Owner Starbase (POS) would, assuming that the defenders loaded the Customs Office with Strontium Clathrates. (If the defenders forgot to do this, then they don't know how to operate a Customs Office.) Once it's in Reinforced Mode, it's temporarily invulnerable and you have to wait for Reinforced Mode to end, by which time the owners will probably have logged into the game, gotten the notification saying that YOU (I think your name is spelled out as being the shooter) attacked it, and mobilized a defensive fleet. Once the Customs Office leaves Reinforced Mode, the defenders can try to shieldboost it while you try to shoot it some more.
Reinforced Mode requires Strontium Clathrates, which can't be loaded into the Customs Office until the shields are boosted back up to... I forget what fraction, but it's no lower than 50%. So, as long as you have a fleet around to defeat the defending fleet, you will only have to wait out the Reinforcement Timer once.
To make a concept clear, here: unless the defenders were negligent in loading the Customs Office with Strontium Clathrates or are ALL on vacation, you cannot completely destroy the Customs Office before the defenders notice and put a fleet in place to protect it when it comes out of Reinforced Mode.
If your own Customs Office is attacked, then the above also applies, but in reverse: you are the defender.
The tax rate on a Customs Office is set by the owner of the Customs Office. Specifically, Customs Offices are corporate, not personal, property. YOU do not own a Customs Office; your player corporation owns it. Making changes to the Customs Office requires the appropriate roles in your player corporation. Corollary to this, you cannot set up a Customs Office with a character in EVE University unless the Directorate has given you roles. You would have to create an alt, then have the alt create a new player corporation, then use that alt to set up the Customs Office. The alt will need the Anchoring skill... I think to at least Level 1. Your alt can make whatever changes are needed to the settings, such as tax rates for different standing levels.