The period always goes outside of the parenthesis, UNLESS you are using more than one sentence in the parenthesis. If you are using more than one sentence, the period DOES still go outside the parenthesis for the LAST sentence. So, for example: blah blah blah (I knew what he had done). blah blah blah (I new what he had done. It was terrible).
"Our Prices Start at $11.99. As Our First Client, Use Coupon Code GET15 to claim 15% Discount This Month!!"