I requested a quote on a set of parts. Got it, it was too expensive in terms of shipping, but it turns out I had only 18g of parts, when shipping would cover up to 100g. Is there a way to move parts back to my cart, so I could add additional 82g of bricks without clicking it all again? I understand some parts might not longer be available or change price. I don't really care, I just want to avoid looking them up again.
No. You cannot put items from quote back to cart, but there is a better option.
As per help topic on Bricklink:
You can add quote items to a wanted list at any time by clicking the Add Quote Items to Wanted List button on the quote summary page
Here how it looks in your quote (just at the bottom of your item list):
Once you click on that button you have an option to add items from your quote to an existing Wanted List or create a new one. This then allows you to have a list of items from quote that you can use to buy in other stores too, not just the one you've requested a quote from.
More details on how to use wanted lists when buying parts can be found in another help article on Bricklink.