need a printable gift voucher for jewelry site.

I need a shopping cart added to my Jewelry sales web site that can issue gift vouchers for certain variable amounts. It is intended the vouchers be issued with a unique number and that a voucher should only be used once which then automatically cancels that voucher number. Printing and management of the vouchers should be done in the management section of the web site. Art work and layout of the voucher must be done and it must be print ready on the management section of the site. Currently a number can be used to achieve this gift certificate, however it is a common number and can be used repeatedly. The voucher Will have a value, and a minimum spend. EG $100 value, minimum spend $110. If the voucher is used for an item worth $90 they would need to pay $20 plus post and packaging to complete the transaction. Both the value and minimum spend should be adjustable by management. There needs to be a post & packaging fee which is also adjustable. There must be space to note the receiver of the voucher. The site already has credit card facilities. That shall remain unaltered. The web site is [login to view URL]