Compounding formula

I'm trying to capture inventery costs per year for a large order. The order will vary, but it could be for 1 year of demand to 15 years of demand.

Assume the order is for 1500 parts and I will only use 300 a year, but pay 2% for inventory costs. At a dollar a part the first year's inventory costs would be $1500 x .02. But the next year I would use 300 parts and only pay inventory costs on 1200 parts and soo on per year.

I need a formula that can add those inventory costs together and give me a total cost of inventory until I run out.

Thank you for your help!