You can sell gift certificates on your SimpleTix site.
On your product list, create each denomination as a product.
Now on the product detail pages mention when the sales of gift certificates end. Mention on the page that we will email them their unique gift certificate code on that date. They will use that gift cerificate code in the "promo code" text box on the shopping cart page.
Once the cut off time comes, go to the Product Sale Report, choose the export button: