An income tax is imposed annually at the rate of five percent on the South Carolina taxable income of every corporation, other than those described in §§ 12-6-540 and 12-6-550, and any other entity taxed using the rates of a corporation for federal income tax purposes, transacting, conducting, or doing business within this State or having income within this State, regardless of whether these activities are carried on in intrastate, interstate, or foreign commerce. The terms "transacting", "conducting", and "doing business" include transacting or engaging in any activity for the purpose of financial profit or gain.