How can I handle a customer who consistently cancels orders?

Our customer base is mostly very good. However, lately, we’ve had a number of cancelled orders before fulfillment. Most are just partial and the customer is very apologetic about the request. The processing charges add up. This led me to look more closely and, in one case, a single customer has placed 18 orders over the last 14 months and has always cancelled everything. Sometimes it’s a few products at a time and other times it’s the entire order but not a single order has ever shipped. (we ship live plants and the customers can request delivery at a suitable time for their weather)

Question1: Are we just being trolled by someone who is bored or vindictive? Maybe she has a shopping addiction but not the budget to match?

Question2: Is she scamming her credit card company for bonus points? There have been a few cards used. Is there a way to report this?

Question3: This is the most important to me right now. Am I legally allowed to withhold a portion of the refund to cover the fees? I’ll add appropriate text to our T&C page, etc. Also, are fees charged on the refund process or just the original purchase?