If you pay the amount demanded by the copyright owner, that will probably be the end of the legal matter, though you need to be sure to get the proper releases. For that reason, I highly recommend you get legal advice from a copyright attorney, at least regarding the releases.
Some of these companies will negotiate a reduced payment. Others will not. I have not heard of any copyright case where they simply went away. There are instances where the copyright owner actually sued if the user of the photo didn’t pay. There may be a threshold amount below which they will not sue. I have only anecdotal evidence in that regard. For sure, you run a high risk of being sued. If you defend yourself, you will spend at least $5000 in legal fees, likely much more than that. If you don’t defend yourself, your opponent will get a default judgment, which means the court will order you to pay money to your opponent. The court will also likely order you to pay their copyright attorney fees.
If you do nothing, depending who your opponent is, you might receive additional letters, both from the complaining company, then from a collection agency and/or a law firm before you get sued.