I wouldn't ignore the letters, because it's possible that should you refuse to pay WalMart could attempt to press criminal charges against you through your local prosecutor or file a civil lawsuit against you. Because the value of the items you attempted to steal were so low in value, it is less likely they will press charges or sue you than if you stole thousands of dollars worth of merchandise.
Perhaps you could call a telephone number listed on the letters, explain your financial situation and you girlfriend's non-involvement and attempt to reach an agreement whereas you would pay a smaller amount of money for Walmart to consider their civil demand settled.
Disclaimer: This does not constitute legal advice. No attorney-client relationship has been formed through this answer.
The demand of a law firm does not create an independent obligation to pay. It is rare, but not unheard of that the company would pursue one for such a small amount. Attention must be paid to any document arriving from a court.
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
If the police weren't called its not likely that you will ever face criminal charges as a result of this (even though they could technically still file a complaint against you within the statute of limitations). I generally advise clients to ignore these civil demand letters. You don't owe them anything. In order for you to owe them something they would have to sue you, (in some jurisdictions prove damages which they most likely couldn't do), and win. Even if they could win, the cost of pursuing this is substantially greater than any amount they can possibly recover so they usually don't. They send out these letters because it doesn't cost much and they are hoping that you don't know better and simply send them the money. Nothing will happen to you if you don't pay it and it will have no bearing at all on whether or not criminal charges will be filed. Id put this behind me and move on.
Don't pay it. The stores lie to you. They tell you if you pay their demand they won't press charges, but they will whether you pay or not. I have never seen a store actually pursue the civil demand past the letter. It's basically a scam.