If the police were not involved, then it's highly unlikely you'll face any criminal charges at this point. The letter they're talking about isn't a fine - it's a civil demand letter.
The letter you will likely get will be from a law firm in Florida that does nothing but send these demand letters. They sound very official and are quite intimidating. They will ask for hundreds of dollars for this incident to "settle" the case.
If you pay, it means you have settled any civil claim for the however much they want. It has absolutely nothing to do with any criminal prosecution. Paying it won't guarantee no criminal charges will be filed, nor will ignoring it cause criminal charges to be filed.
If you ignores the letter (and the second, third and fourth that are likely to follow, each sounding more and more urgent and threatening), then they must make a choice - file a civil lawsuit against you or let it go.
I have yet to speak with an attorney whose client has been sued in a situation like this. The law firm that sends these letters operate on volume. For the price of a form letter and a stamp, if they collect several hundred dollars, it's a huge windfall for them. Based simply on the volume of letters they send out, if even a fraction pay their outrageous demands, they collect large sums.
Let's say they actually do file a suit against you. You can always choose to settle it later or fight it. If you fight it, the store would have to prove their "damages." The item was recovered and I presume was able to be resold. Damages = $0. The store personnel that were on duty were already on the clock, so presumably the store didn't incur any overtime costs above and beyond what they already pay their employees. Damages = $0. Even in the best case scenario for the store, let's say the court awarded the hourly wage for two store employees for two hours worth of time to deal with this situation. What do they make? $20 an hour each? Total damages (even in the best of all worlds) - $80... far less than the hundreds of dollars the law firm will demand.
If you pay in full, it goes away. If you offer to pay half (or some other lesser amount), it potentially opens the door to further harassment by them.