by Jim » Thu Aug 20, 2009 12:35 pm
You can't manipulate the cart totals. That information is stored in the shopping cart for the purchase and is not directly modifiable. This is a security feature for the merchant so a shopper can't manipulate the prices of items in the cart and purchase items at a price other than what the merchant has listed.
I really doubt that you could add javascript that would add the difference between what the shopper placed in the cart and $20. But if it were possible it might be done by adding a Variable Price product for the difference. I don't know javascript well enough to know how that could be done, if at all.
The warning that ShopSite displays tells them that they haven't met the minimum and allows them to make the decision if they want to add additional items or not.
As a side comment, If I were a shopper and had the total automatically increased to something other than what I put in the cart I would abandon the cart and go elsewhere.