This is what I added to goods.ini

[Good
nickname = than_battle_hull
category = shiphull
ship = br_battleship
;Note the original ship name is left alone
ids_name = 237005
item_icon = Equipment\models\commodities\nn_icons\li_fighter.3db

[Good
nickname = than_battle_package
category = ship
hull = than_battle_hull
addon = infinite_power
addon = ge_s_scanner_02
addon = ge_s_tractor_01
addon = ge_bb_engine_01
addon = sfx_rumble_battleship
addon = br_battleship_turret01, HpTurret_B1_01
addon = br_battleship_turret01, HpTurret_B1_02
addon = br_battleship_turret01, HpTurret_B1_03
addon = br_battleship_turret02, HpTurret_B2_01
addon = br_battleship_turret02, HpTurret_B2_02
addon = br_battleship_turret02, HpTurret_B2_03
addon = br_battleship_turret02, HpTurret_B2_04
addon = br_battleship_flak_turret01, HpTurret_B4_01
addon = br_battleship_flak_turret01, HpTurret_B4_02
addon = br_battleship_turret04, HpTurret_B4_03
addon = br_battleship_turret04, HpTurret_B4_04
addon = br_battleship_turret04, HpTurret_B4_05
addon = SlowLargeRed, HpRunningLight19
addon = SlowLargeRed, HpRunningLight20
addon = SlowLargeRed, HpRunningLight09
addon = SlowLargeRed, HpRunningLight10
addon = SlowLargeRed, HpRunningLight11
addon = SlowLargeRed, HpRunningLight05
addon = LargeWhiteSpecial, HpHeadLight01

And this is what I added to market_ships.ini

marketgood = than_battle_package, 1, -1, 1, 1, 0, 1, 1
;marketgood = gf2_package, 4, -1, 1, 1, 0, 1, 1
;commented out the original good

I also added several lines to the original shiparch.ini to make the camera work well. But I don't understand why all the other tutorials have me add a new entire entry into shiparch.ini. I'm wondering if there is a reason for it?

Yes, because if u only 'change' an entry u might screw another ship up witch might lead to crashes nd so...