Attributes with a discount and one-time charge

I'm using Zen Cart 1.3.9h and I'm stuck.

I want to add an attribute that will add a set dollar amount for the attribute and a percentage discount on the product.

It's for fundraising purposes: Buy this product at regular price, or support the cause by purchasing a 5% discount for $5 dollars.

I keep trying to adjust the *Price Factor *and *Offset *using both positive and negative numbers, which appear to show the effects of the changes when viewing the product in edit mode, but when I add it to a shopping cart, I either get the same regular price for the product, or an *increase *in price for the product.

The only thing that works correctly for me is the One Time price for the attribute.

Can someone tell me what I'm doing wrong? Perhaps a better explanation of Price Factor and Offset would help me. It's not clear to me what they are or how they work.

I finally found the answer to my question with the helpful explanation of Factors and offsets in this post: http://www.zen-cart.com/forum/showthread.php?t=66801

Under price, change the '+' to '-' to get the discount.
Leave the price at 0.00
Set the One Time value the cost of the attribute.
Set the Price Factor to the pecentage you want the customer to pay.
Set the Price Offset to '1'

The attribute will subtract the percentage from the product price and add the attribute cost to the total.