I have a small question. I know it's small (yes petty!) but it's bugging me

Right now I'm taking advantage of the current no selling fees offer and I've listed a few items priced a good deal lower than I would normally because I want to de-clutter. I charge a low domestic shipping cost for standard registered post, too low to cover the postal charges much less my high quality, careful packaging.
A high feedback domestic (Australian) buyer pounced on a pair of NIB Christian Louboutin shoes (at a great price) but then came back a few hours later, after paying, to query shipping. She wants an upgrade to Express with Signature and is prepared if necessary to pay the 'extra three bucks' she says will cover the upgrade. Well it costs a lot more than three dollars to Express Post + Signature a heavily padded box of shoes interstate these days, and I'm fairly sure she knows this.
I want to let it go and give her the upgrade without charge but the shoe price was so low that I'm not in the mood to be more generous - I think I've reached my limit! And mostly it bugged me that she hadn't contacted me about shipping costs prior to purchase as per my listing - "
please ask questions before buying" (lol I'm feeling so grumpy with buyers atm)
Should I graciously and generously upgrade without further cost to her, or invoice her for some or all of the extra cost? She is prepared to pay about 1/3 of what the actual cost will be, by my estimation with size and weight and distance etc.
Thank you for reading!