Buyer wants to cancel, then demands cheaper shipping

[3islanders]

Buyer used Buy It Now, and paid.  Messaged me a few hours later, twice, asking to cancel because she "didn't realize" the shipping cost more than the item. The shipping would have been stated on the page, and she PAID. So probably buyer's remorse.  I told her to go ahead and request to cancel, and unpacked her item. Then she messaged again disputing the shipping cost. I had used calculated shipping, within Canada, weighed and measured the box before listing, and eBay still came up with a cheaper price than what it would cost me to mail it. Buyer doesn't believe me, demands weight and dimensions so she can calculate the shipping herself.

Now she's asking for cheaper shipping rates via a different service. It's a perishable item so I am not offering slow regular parcel.

I don't see this going well -- given that she first sent 2 requests to cancel, am I OK to go ahead and do it?  She didn't use an official link from her end, just a message asking to please help her cancel. Then all the back-&-forth started.  Augh, I forgot what a hassle selling on here is  >:[

What do I do now?

[whistlersmother1]

i find ebays shipping is wrong about half the time. it has even managed to give free shipping a couple of times. this is great as it can be more that the item was sold for. also a lot of their shipping dates to various areas is no longer accurate also. shipping to china even with the letter registered is taking 90 days. i have to give a refund only to have it arrive but ebay has already erased the sale and it has no way for the buyer to reimburse me.  i like some of their changes but they are getting sloppier by the day.