Cash or a refillable cash card.
I withdraw $100 at a time in cash from our joint bank account. If she asks the money went for trivial things like lunch and stuff for my shop as much of it does. I am fortunate that I get a few dollars cash from work spiffs too. For the most part I buy in brick and mortar stores for items she does not need to know about.
We are in a semi-DADT relationship and if something comes in from Amazon she does not ask about it, and if she does I just tell her I will show her when the time is right. She knows about my shoe fetish, panties, and lingerie. I am confidant I could order pretty much anything and have it shipped to the house and she would not worry about it.