Can Donate page show amounts already in cart, and then update those amounts rather than always add more donations?

Issue Summary

I have a client who is insisting that if donations were previously added to the cart, that the Donate page Amount field show the previously entered value, and when submitted, update the values to the new values entered, even if they are zeroing out existing items.

I'm not fond of the idea of implementing this because I expect that any implementation will be a one off as I don't think most administrators want it to function this way.

Unfortunately, I'm still wading through Drupal and Ubercart documentation and the methods for implementing this are currently well beyond my capabilities. I'm hoping someone will be able to point me in the right direction and possibly provide some example code.

If this is just a really dumb idea, I'd appreciate hearing a reason why.