Petitioner's marriage certificate

I petitioned for my parents, and now the NVC is asking for my marriage certificate?! Why would they need my marriage certificate for my parents’ visas? Even if they do need it, and I upload a copy online, I believe they still require original documents at the time of the interview. It’s a hassle to get two certified copies of my marriage certificate from California and then send them to two different countries (my parents live in separate countries). Any advice?