We store credit cards with our payment gateway provider, one of the world's foremost payment processing companies, and never keep them on our own systems. All systems involved follow the rigorous PCI-DSS security protocols, are regularly penetration tested, and constantly improved.
There should be no more concerns storing a card with us than with any major e-commerce provider. We are also happy to invoice storage charges if you wish, and you can send us a check or wire for payment of the balance. You can even pre-fund storage fees in the same way... just ask customer service.