Creating a product for shipping upgrades?

Hi - I'm wondering how people handle shipping upgrades?

Most people choose USPS Priority Mail for shipping, and many people want to upgrade shipping. I created a "product" with a flat price for upgrading to Express Mail, but ideally, I'd have a product that was more flexible based on the shipping costs.

I charge $6 for Priority Mail, so I experimented with a product that was -$6 (and then shipping costs would be added to that) but a product with a price of -$6 just looks weird.

Does anyone have any solutions? Thanks.

Look at and use the USPS ship module
Select the desired & appropriate methods

I am using the USPS shipping module, but there isn't a way (that I can see) that helps me with my shipping upgrade question.

The basic problem is that if someone has already paid for one shipping method on a previous order (e.g. USPS Priority Mail), and now wants to upgrade, how can you automate that and have the customer upgrade via the shopping cart, instead of having to manually doing the subtraction (since they should just pay the incremental shipping cost).