There are no storage or maintenance fees for the bullion that you have transferred to your Stacker Market™ account.
Related articles:
What is the cost to deposit my own bullion into Stacker Market™?
http://help.stackermarket.com/en/articles/5355606-what-is-the-cost-to-deposit-my-own-bullion-into-stacker-marketCan I deposit bullion I already own into my Stacker Market™ account?
Can I move bullion from another vault to my Stacker Market™ account?