I want to add gift certificate processing to my website, www.jerrys-ebooks.com .

I have a category "gift certificates" add a subcategory gift certificate denominations.
The purchasing aspect of gift certificates is mostly taken care of – need to address the distribution process of purchase of gift certificate….
still thinking about that
 - Ideally the download process would be by passed, and simply an email sent to the userid of the certificate with details of the purchase 
- ie. certificate no., date issued, user-id, amount, possibly a pic of a gift certificate - (as well as the cert.no being displayed on the screen.)

I need to add the claiming of gift certificates, to the payment process 
(and possibly the initial distribution process of a gift certificate, so that it is automated)).

ie.
checkout_payment.php
checkout_confirmation.php
(I think these 2 cover it)



In addition, a user may only want to use, say $20 of a $50 certificate, so they can use the certificate again until the balance is zero.
I'd like am admin form where I can enter the details and have the new sql table updated. 
This allows me to issue certificates for promotions, to family, friends etc). 
I can add the password to the form / php page myself - can use hosting capability of passwording a directory.

I'd imagine a sql table with the following data elements:- 

cert-number  = next number in sequence
cert-issued date  = todays date
registered-user-id  = signed in user id or admin
cert-amount-issued  = $5, $10, $20, $50 or $100
cert-amount-remaining
cert-balance  = starts at issued amount 
cert-indicator .... where "b" = indicates a bought certificate and  "a" = indicates admin issued


I'd imagine the confirmation page (or earlier?) has a an extra lines and 2 input fields, something like 
"If you have a gift certificate and want to use some or all of it "
" enter certificate no"  "enter amount to use"


The sort of things to error check in the confirmation stage is: -

•       certificate no. exists 
•       certificate hasn't expired (good for 6 months ie. Add 6 months to issue date in Julian?)
•       remaining cert-amount>= amount user input "to use"
•       entered cert-amount <= total of selected items 

Points
•       if the item purchased is a gift certificate, don't display confirmation page with certificate option…ie
leave the display as it is now
•       Update the purchase total, minus the gift cert. amount 


Note: I will be not be able to get internet access between Oct 6-13..pls send questions before or after these dates.