Here's the standard for service of a custody matter, including a petition to modify:
Service is the delivery of court papers to a plaintiff or a defendant. Court papers may be served by mail, but you must follow very specific rules. Court papers may be served by personal service - handing a copy of the paper to the other party – but not by you or a person related to you. Proof of Service, called Affidavit of Service or Certificate of Service, is a form you must file with the court explaining how court papers were delivered to the other party. A court will not act on your case unless you have served the court papers and filed proof that you followed Rules of Court.
Pa. R.C.P. 1930.4 states the options for service of original process, the custody or divorce complaint. Pa. R.C.P. 440 states the options for service of legal papers other than original process.
Please use the appropriate affidavit of service form for the mode of service you will use. If you plan to serve copies of your legal papers by mail, please use form 1. If you use hand deliver service, perhaps by a deputy sheriff or someone else not you or related to you, use form 2. If your situation requires you to use other means of service different from original process, use form 4. You will need to consult with the Pennsylvania Rules of Court regarding service in your type of case to ensure you are properly following the rules.
This handy document from the Bucks County Prothonotary will also help.
But, what is a person is to do if the mother/father of their child is in prison? If the person is in the Bucks County Correctional Facility, you will have to get special service from the Bucks County Sheriffs. Your Bucks County divorce lawyer should know that you can't just mail it to the prisoner. Even though, they are literally "captive", it doesn't matter. You have to have special service. Make sure to include the custody petition, order, and Criminal Record/Abuse History Verification in your service. The cost (as of January 2016) is $79.