Just about every store I patronize has on-line ordering. If I want I can buy a gift card with cash at any retail store. There are kiosks all over. I buy gifts cards at my local grocery store for stores I normally patronize because I get fuel points towards gasoline purchases. I also get 1% back on the grocery store rewards card, and, another 1% back when charged to my Visa or Mastercard. Since you want to fly under the radar I would not recommend doing what I do. Macy's, JC Penny, Target, Kohl's have order on line and pickup at the store. I order dresses and lingerie on line and pickup at the 'will call' desk. You can pay with the store gift card. Many stores also allow the purchaser to also list another person authorized to pickup the package. If you're too shy to pickup ladies' wear, then order under your wife's name with your name as an authorized person to pickup the order.
As others have stated you may order on Amazon with its gift card and have it delivered to a lock box with a single use access code.
As to the DADT you state your wife does NOT want to see you. DADT implies a wife knows which is totally different than a wife having zero knowledge. I can understand not ruffling her feathers too much, but, sneaking around would be way too stressful for me. If you have your separate VA disability check then financial considerations are not an issue. My wife knows I am a cross dresser. She is an ostrich when it comes to my cross dressing. Not a snide comment or a bad word since the mid 1980's. All our accounts are joint. All our funds and credit cards are joint. She never checks the credit card statements. That annoys me. Basically she has no clue about our finances. I have to leave her a road map just in case I fail to wake up in the morning.......dying in my white nightie and panty. Level with your wife and tell her you need a little more freedom to pursue your cross dressing. Negotiate some boundaries or come to a better understanding of each other's needs.