Assuming the coins don't have to be returned once I purchase them? (as it's a gemach?)or I guess I will call and see for myself.
Thanks
You don't have to return. It's a non profit, they sell it to you for the price of silver at the time they bought it.
When I made a pidyon haben, silver had jumped dramatically in the few months before, and they gave me the (much lower) price that they bought it at, a year before.