I've contacted my bank, I've spoken with jagex payment support, and NOTHING is working. For the last several months, I've had to buy membership through Amazon. I was hoping I could get this issue fixed so I could get premier club. Payment support tells me to call my bank, and my bank tells me there's nothing wrong with my account. I've even tried purchasing through paypal and contacting mastercard!
If you have already been in contact with Billing support, there probably isn't much we can offer in the way of advice in here.
Try clearing your browser cache then try the payment system again. This is in case there is a technical problem between your computer and the Jagex computer.
Instructions:
How to clear your browser Cache
If you have another credit card, try it instead e.g. try a VISA instead of a Mastercard.
Is your card linked to a bank account or is it a pre-paid card?
PayPal won't work if the card isn't linked to a bank account.
Pre-paid cards might not work if you aren't in the UK.