You make a reservation like normal (with the promo code "BCFD," although I don't know that that is absolutely necessary). You pay taxes and fees on whatever daily rate your reservation is at.
For example, if you make a reservation for ANC at $20 per day for four days and had four certificates, the certificates would cover the $80 base rate. You would then pay taxes and fees as follows:
8% city rental tax: $80 x .08 = $6.40
10% state rental tax $80 x .1 = $8.00
11.11% airport concession recovery fee: $80 x .1111 = $8.89
Consolidated Facility Charge: $4.87/day x 4 days = $19.48
Total bill for you: $42.77
If you rent from ANC in the summer and your reservation comes in at $150/day, your tax bill would be $194.14--not cheap, but certainly better than the $794.14 it would otherwise be!