DV = 'debt validation' or 'validation of debt'. It's the process whereby you make them prove that you owe what they think you do, that they have a right to collect it, etc. There's a good template for that letter
here. Send it certified mail, return receipt requested, and hang onto the little green card you get back. That's your proof that they got the letter.
FDCPA = Fair Debt Collection Practices Act. That's the federal law that governs how a debt collector can proceed when trying to collect a debt, the things he can and can't do, etc.
In particular, I'd like to know what this $200 they claim you owe is for... Doesn't sound as though it's for goods purchased. And Wal-Mart LP doesn't have the authority to levy a fine or other punitive measure on you. Only a court of law can do that.
Like I was saying, I think you have cause for legal action. As for an attorney, you can likely get a consultation for free. Call around. You might also check with your local legal aid outfit. Another good possibility is to see what legal assistance mmight be available through your husband's unit. I'm decades outta the loop, and the acronyms keep changing, but I'd bet you a cup of coffee that help is available. The military has historically taken a really dim view of this sort of thing.