What I'm asking is, do you actually owe the money?
Usually, if there is to be a court case, a certified letter is not sufficient. One usually must be served in person (service on your father would be ok), and served with the court complaint.
If this is JUST a letter at this point, it's likely no court case has been filed against you yet.
If it's a complaint with a court case number, than it probably is a case, in a PA court. You can easily check by calling the court (or in some instances, you can look online).
If it's a complaint, you must respond or you can lose by default. You would send the court an original response with a copy (or two), and a self-addressed stamped envelope. You would use a cover letter to the court clerk, and ask that they "conform" your copy (ie, datestamp your copy) and mail the copy back to you in the SASE. You would then make a copy of that and send it to the complainant (or if you mail in two copies, you would then have one to send to the complainant).
The reason that I'm asking if you really owe the money is there seems to be a nation-wide rash of false claims being made for supposed old credit card debt. Some of these claims are completely false. Some are real, but are out of date, or inflated, etc.
If this is a court case, you can contact the court clerk's office and make arrangements to "appear" by telephone or video conferencing should you want to contest this. (You can also ask to "appear" for the sole purpose of arguing that service upon you was not properly made).
I'd be happy to write some more today (I won't be here Friday, but will be available Saturday). Just write back!
Or, if I've answered your question to your complete satisfaction, please click on "accept." (We can keep writing even after you accept, if you think of something else you want to write about, as long as it's related to your original question).
All my best,