If you've received a discount code, you will be prompted to enter it in AFTER you go through check out and BEFORE you finalize the amount paid.
The discount will be applied to the original purchase amount, which does not include taxes and shipping. Tax is calculated on the price AFTER the code is applied. Shipping is also added to the amount, and must be paid.
If you have any questions, or need assistance with using your discount code, please contact firstname.lastname@example.org.