She found me this way and I'm not a permission seeking type, nor one to really explain/justify. I'll boast about good deals, but she's very much in the dark.
One strategy thats worked well was taking away her credit card and getting her a card under my amex. She gets to tease about it to her friends how she charges everything to my card, but when it comes down to significant purchases, she seeks my permission.
I on the other hand monitor everything she buys, get veto rights on her purchases and answer to no one as to my purchases.
Check and mate.