You received a civil demand letter, an offer to settle a possible claim against you. It is Just an offer.
In Kentucky, I had hundreds of clients who received such a letter and a few chose to pay it. If you do pay it, it does not affect your criminal case either way. Also, if you settle this matter without their resorting to a court case, they cannot bring such a claim later as the settlement should stop them.
On the other hand, if you choose not to pay their demand, they would have to prove their damages for restitution, and, as you've written, they may not be able to do that.
This is general informational response is based only on the information given. It should not be relied upon without consulting a lawyer and getting a full consultation. This response to the question does not create an attorney-client relationship.
The general agreemetn among Avvo attorneys is that a civil demand letter creates no independent obligation to pay. You should give special attention to anything which arrives from a court or police department, but a letter from a law firm, without the filing of a civil lawsuit, is a different matter.
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.
The alleged shoplift opens you up to potentially both criminal and civil liability. In the criminal arena, Washington has a misdemeanor compromise statute which provides you can pay the retail establishment what they want and your case will be dismissed (court approval needed). There are some significant limitations on the application of the statute. Make sure you discuss that with your attorney. In terms of civil liability, the store could sue you civilly for their "damages" (i.e. the amount of money they are out b/c of the alleged theft). There are issues with double recovery depending on what happens in the criminal case. Definitiely talk to your lawyer about the potential civil ramifications.
They aren't combining the amounts, they are charging you 5x the amount or some other percentage. 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.