I for one hate tacked on fees, you've stated you know you can get the job done so just do it, do a great job and you'll be rewarded with future work. The only time I'd charge an "urgent fee" would be if I had to pay extra staff to get it done on time but I'd be upfront with the customer and make sure he's aware of the extra expence involved to get his job done. Again though the PR on getting the job done without the extra charge should return you a loyal customer.

Regards Steve.