I'm going to give you the perspective of a former shopping addict.
I did things that were very similar. My husband kept bailing my credit cards out, sometimes sacrificing his bonus to do it. I could keep a lid on my behavior for a while, but I'd always go back to overspending on very similar categories to your wife.
It wasn't that I was TRYING to take advantage of my husband, and it certainly wasn't that I didn't appreciate him. It was really just that I felt like somehow having THINGS made me better, more likable, more attractive. It almost wasn't about him at all. I promise you you are barely a thought in this equation. Of course that is not right.
What made me start to turn it around was exactly what people are recommending here - a come to Jesus meeting. One day he looked at me and said, "I can't trust you, and that makes me really sad."
That was like a knife in my stomach. Somehow I blurted out that I never meant it to hurt him -- and it was totally true. I figured I had to straighten up.
But you know what? It is super hard. I started reading clinical books on overspending. I considered group treatment for compulsive spending (and would have gone, if I couldn't get it right). I changed things like purging my credit card numbers from websites (it's really easy when all you have to do is push 'buy' with stored information). Sometimes I fell down. I had to make myself use the tracking app I bought. Sometimes my strategies didn't work...I tried to pay all cash but ended up just not being aware of where my cash went.
What finally enabled me to be successful was the idea that it was him or that Oscar de la Renta dress. What good would the dress do me if I was alone and had nowhere to go?
The first real step was setting up alerts on my credit cards so that every time I used it, he would get a ping. Every time I pulled that out of my wallet or went to buy something I knew that it would tattle on me. And that was negatively reinforcing.
The setup we have now is that we both have allowance and a joint account. I became involved in budgeting, so I could see the damage I was doing firsthand. Previously this didn't work because I would always exceed my allowance, so I'm improved. I am aware of my triggers though - I can no longer go shopping without a list (because I'll buy whatever) and sale season tends to be bad. I had to unsubscribe to all 'deal' emails.
It has been hard but at the bottom line I love my husband more than I love designer clothes and accessories. Maybe you just want to ask her which she prefers.
If you like you can PM me and I can tell you the books that helped me. Although she has to read them, and she has to come to 'Jesus' herself. I never opened up cards he didn't know about...that's pretty serious. She probably can't do it without a group, maybe Wellbutrin (shown to reduce compulsions), and certainly she should work with a therapist to replace the bad behavior with something good.