I bought an air tank from a kid off the nation. The deal was i pay $85 and he pays shipping. Now i needed the tank really bad so he was gonna send usps priority (2 day) for around $10 according to their website. I told him if priority doesnt deliver on saturdays then do express. I asked him today and he said he sent priority and it cost $50. He wants me to pay $25 of it, but the original agreement was $85 shipped. Should i give him the money?